®i{e  ^.  ^.  pm  pfararg 


*QH50l 


SCIENCE. 


C,  179"). 


r 


So- 
rt rs  of 
ud-'9  : 


It... 


* 

# 


-If 
•If 


ARCHIBALD  A.  CALDWELL,  Salisbury 
ROBERT  D.  DICKSON,  Wilmington, 
EDWARD  B.  DUDLEY, 
JOHN  H.  DILLARD,  Rockinqliam, 
JOHN  S.  ERWIN,  Burke, 
SAMUEL  HALL,  Wilmingtuu, 
JOHN  D.  HAWKINS,  Franklin, 
WILLIAM  J.  HAWKINS,  " 
HENRY  C.  LOGAN,  Halifax,  Va. 
JAMES  A.  LONG,  Randolph, 
HORATIO  L.  POLK,  Fayette  Co.  Tenn. 
THEOPHILUS  SCOTT,  Raleigh, 
HENRY  A.  SY'DNOR,  Halifax,  Va. 
ROBERT  STRANGE,  Fayelleville. 
JAMES  F.  TAYLOR,  Ralcigli. 

3 


.St. 

if- 


* 


.^ 


i^ssa^ 


"^ 


^•■^« 


i 


i. 


This  book  must  not  be 
taken  from  the  Library 
building. 


.}/ 


'  iy  ^  *>«> 


THE  BRIDGEWATER  TREATISES 

ON  THE  POWER,  WISDOM,  AND  GOODNESS  OF  GOD, 
AS  MANIFESTED  IN  THE  CREATION. 


TREATISE  V. 

ANIMAL  AND  VEGETABLE  PHYSIOLOGY,  CONSIDERED 
WITH  REFERENCE  TO  NATURAL  THEOLOGY. 

BY  PETER  MARK  ROGET,  M.  D. 

SEC.  K.  S.  ETC. 


IN  TWO  VOLUMES. 
VOL.  I. 


**  Ask  kow  the  beasts,  and  they  shall  teach  thee;  and  the  fowls  of 
the  air,  and  thet  shall  tell  thee: 

"  Or  speak  to  the    earth,  and    it  shall  teach  thee;    and    THE  FISHES  OF 

the  sea  shall  declare  tjnto  thee. 

"  Who  knoweth  not  in  all  these  that  the  hand  of  the  Lord  hath 
WROUGHT  this?"  Job,  xu.  7,  8,  9. 


ANIMAL 


AND 


VEGETABLE 


PHYSIOLOGY, 


CONSIDERED  WITH  REFERENCE 


TO 


IVATTRAIi   THEOL.OGY. 


BY 


PETER   MARK   R  0  G  E  T,    M.  D. 

SECRETARY   TO   THE  ROYAL   SOCIETY,  FULLERIAN   PROFESSOR  OF  PHYSIOLOGY  IN   THE  ROYAL 

INSTITUTION  OF  GREAT  BRITAIN,  VICE  PRESIDENT  OF  THE  SOCIETY  OF  ARTS, 
FELLOW    OF  THE    ROYAL   COLLEGE    OF    PHYSICIANS,   CONSULTING  PHYSICIAN,  1*6    THE   QUEEN 
charlotte's  lying-in  HOSPITAL,  AND  TO  THE  NOKTHERN 
DISPENSARY,  ETC.  ETC. 


VOL.  I. 


PHILADELPHIA: 
CAREY,  LEA  &  BLANCHARD. 

1S36. 


GRIGGS  6l  CO.,  PRINTERS. 


TO  HIS  UOTAL  HIGHNESS 

PRINCE    AUGUSTUS    FREDERICK, 

DUKE    OF    SUSSEX,    K.  G. 

PHESIDEXT  OF  THE  BOYAL  SOCIETY, 

&c.  &c.  &c.  &c. 
THIS  TREATISE 

IS,  WITH  PERMISSION,  HUMBLY  DEDICATED, 

AS  A  TRIBUTE  OF  PROFOUND  RESPECT  AND  GRATITUDK 

FOR  THE  BENEFITS  RESULTING  TO 

SCIENCE 

AND  ITS  CULTIVATORS, 
FROM  HIS  ILLUSTRIOUS  PATRONAGE, 

BV  HIS  DEVOTED  HUMBLE  SERVANT, 

P.  M.  ROGET. 

•^    4k : 


V 


?^1990 


PREFACE. 


I  PROBABLY  never  should  have  ventured  to  engage 
in  the  composition  and  publication  of  a  work  like  the 
present,  had  not  that  task  been  assigned  me  by  my 
nomination   as  one   of  the  writers  of   the   series  of 
Bridgevvater  Treatises,  and  had  I  not  deeply  felt  the 
honour  done  me  by  that  appointment,  as  well  as  the 
importance  of  the  duty  which  it  imposed.    The  hope, 
in  which  I  have  indulged,  that  my  labours  might  even- 
tually be  useful,   has  been  my  chief  support  in  this 
arduous    undertaking;    the    progress   of    which    has  • 
throughout  been  seriously  impeded  by  the  various  in- 
terruptions incident  to  my  profession,   by  long  pro- 
tracted anxieties  and  afflictions,  and  by  the  almost 
overwhelming  pressure  of  domestic  calamitv. 

The  object  of  this  treatise  is  to  enforce  the  great 
truths  of  Natural  Theology,  by  adducing  those  evi- 
dences of  the  power,  wisdom,  and  goodness  of  God, 
which  are  manifested  in  the  living  creation.  The 
scientific  knowledge  of  the  phenomena  of  life,  as  they 
are  exhibited  under  the  infinitely  varied  forms  of  or- 


VIU  PREFACE. 

ganization,  constitutes  what  is  usually  termed  Physi- 
ology, a  science  of  vast  and  almost  boundless  extent, 
since  it  comprehends  within  its  range  all  the  animal 
and  vegetable  beings  on  the  globe.  This  ample  field 
of  inquiry  has,  of  hite  years,  been  cultivated  with  ex- 
traordinary diligence  and  success  by  the  naturalists 
of  every  country  ;  and  from  their  collective  labours 
there  has  now  been  amassed  an  immense  store  of  facts, 
and  a  rich  harvest  of  valuable  discoveries.  But  in 
the  execution  of  my  task  this  exuberance  of  materials 
was  rather  a  source  of  difficulty ;  for  it  created  the 
necessity  of  more  careful  selection  and  of  a  more  ex- 
tended plan. 

In  conformity  witli  the  original  purpose  of  the 
work,  which  I  have  ail  along  endeavoured  to  keep 
steadfastly  in  view\  I  have  excluded  from  it  all  those 
particulars  of  the  natural  history  both  of  animals 
and  of  plants,  and  all  description  of  those  struc- 
tures, of  which  the  relation  to  final  causes  cannot 
be  distinctly  traced;  and  have  admitted  only  such 
facts  as  afford  manifest  evidences  of  design.  These 
facts  I  have  studied  to  arrange  in  that  methodized  or- 
der, and  to  unite  in  those  comprehensive  generaliza- 
tions, which  not  only  conduce  to  their  more  ready 
acquisition  and  retention  in  the  memory,  but  tend 
also  to  enlarge  our  views  of  their  mutual  connexions, 
and  of  their  subordination  to  the  general  plan  of  cre- 
ation.    My  endeavours  have  been  directed  to  give 


PREFACE.  ix 

to  the  subject  that  unity  of  design,  and  that  scientific 
form,  which  are  generally  wanting  in  books  profes- 
sedly treating  of  Natural  Theology,  published  prior 
to  the  present  series ;  not  excepting  even  the  unri- 
valled and  immortal  work  of  Paley.     By  furnishing 
those  general  principles,  on  which  all  accurate  and 
extensive  knowledge  must  substantially  be  founded,  I 
am  not  without  a  hope  that  this  compendium  may 
prove  a  useful  introduction  to  the  study  of  Natural 
History;  the  pursuit  of  which  will  be  found  not  only 
to  supply  inexhaustible  sources  of  intellectual  grati- 
fication, but  also  to  furnish,  to  contemplative  minds, 
a  rich  fountain  of  religious  instruction.     To  render 
these  benefits  generally  accessible,   I  have  confined 
myself  to  such  subjects  as  are  adapted  to  every  class 
of  readers;  and,  avoiding  all  unnecessary  extension  of 
the  field  of  inquiry,  have  wholly  abstained  from  en- 
tering into  historical  accounts  of  the  progress  of  dis- 
covery; contenting  myself  with  an  exposition  of  the 
present  state  of  the  science.    I  have  also  scrupulously 
refrained  from  treading  in  the  paths,  which  have  been 
prescribed  to  the  other  authors  of  these  treatises;  and 
have  accordingly  omitted  all  consideration  of  the  hand, 
the  voice,  the  chemical  theory  of  digestion,   the  ha- 
bits and  instincts  of  animals,  and  the  structures  of  an- 
tediluvian races  ;   the  extent  of  the  field  which  re- 
mained, and  which,  with  these  few  exceptions,  em- 
braces nearly  the  whole  of  the  physiology  of  the  two 
Vol.   I.  B 


X  I'KEFxVCE. 

kingdoms  of  nature,  already  affording  ample  occupa- 
tion for  a  single  labourer. 

The  catalogue  of  authors  whose  works  have  fur- 
nished me  with  the  principal  facts  detailed  in  these 
volumes,  is  too  long  for  insertion  in  this  place.  I 
have  not  encumbered  the  pages  of  the  Vv^ork  by  con- 
tinual citations  of  authorities ;  but  have  given  refer- 
ences to  them  only  when  they  appeared  to  be  parti- 
cularly requisite,  either  as  bearing  testimony  to  facts 
not  generally  known;,  or  as  pointing  out  sources  of 
more  copious  information.  It  may,  how^ever,  be  pro- 
per to  mention,  that  I  have  more  especially  availed 
myself  of  the  ample  materials  on  Comparative  Anato- 
my and  Physiology  contained  in  the  works  of  Cuvier, 
Blumenbach,  Cams,  Home,  Meckel,  De  Blainville, 
Latreille,  and  St.  Hilaire,  and  in  the  volumes  of  the 
Philosophical  Transactions,  of  the  Memoires  and  An- 
nales  du  Museum,  and  of  the  Annales  des  Sciences 
Naturelles.  I  should  be  ungrateful  were  I  not  also 
to  acknowledge  the  instruction  I  have  derived  from 
my  attendance  on  the  lectures  at  the  Royal  College  of 
Surgeons,  delivered  successively,  during  many  years, 
by  the  late  Sir  Everard  Home,  Sir  Astley  Cooper, 
Mr.  Lawrence,  Mr.  Brodie,  Mr.  Green,  and  Sir 
Charles  Bell;  and  also  from  those  of  Professor  Grant, 

I 

at  the  University  of  London. 

I  have  likewise  to  return  my  thanks  for  the  liberal 
manner  in  which  the  Board  of  Curators  of  the  Hunte- 


PREFACE.  Xi 

rian  Museum  gave  mc  permission  to  take  such  draw- 
ings of  the  preparations  it  contains^  as  I  miglit  want 
for  the  illustration  of  this  work;  and  to  Mr.  Clift,  tlic 
conservator,  and  Mr.  Owen,  the  assistant  conservator 
of  the  museum,  for  their  ohliging  assistance  on  this 
occasion.  Mere  verbal  description  can  never  con- 
vey distinct  ideas  of  the  form  and  structure  of  parts, 
unless  aided  by  figures;  and  these  I  have  accordingly 
introduced  very  extensively  in  the  course  of  the 
work.^ 

Being  compelled,  from  the  nature  of  my  subject, 
and  in  order  to  avoid  tedious  and  fatiguing  circum- 
locution, to  employ  many  terms  of  science,  I  have 
been  careful  to  explain  the  meaning  of  each  when  first 
introduced:  but  as  it  might  frequently  happen  that, 
en  a  subsequent  occurrence,  their  signification  may 
have  been  forgotten,  the  reader  will  generally  find  in 
the  index,  which  I  have,  with  this  view,  made  very 
copious,  a  reference  to  the  passage  where  the  term  is 
explained. 

I  beg,  in  this  place,  to  express  my  deep  sense  of 
the  obligation  conferred  on  me  by  Mr.  Davies  Gil- 
bert, the  late  president  of  the  Royal  Society,  to  whose 
kindness  I  owe  my  being  appointed  to  write  this 
treatise. 

*  All  the  wood  engravings  have  been  executed  by  Mr.  Byfield,  and  the 
drawuigs  for  them  were,  for  tlie  most  part,  made  by  Miss  Catlow,  whose  as- 
sistance on  this  occasion  has  been  most  valuable  to  me. 


XU  PREFACE. 

I  also  take  this  opportunity  of  conveying  my  best 
thanks  to  my  friend  and  colleague,  Mr.  Children,  of 
the  British  Museum,  for  his  kind  assistance,  in  re- 
vising the  sheets  while  the  work  was  printing,  and 
for  his  many  valuable  suggestions  during  its  progress 
through  the  press. 

A  catalogue  of  the  wood  engravings  has  been  sub- 
joined; and  also  a  tabular  view  of  the  classification  of 
animals  adopted  by  Cuvier  in  his  ^^  Regne  Animal,'' 
with  familiar  examples  of  animals  included  under  each 
division;  both  of  which  I  conceived  might  prove  use- 
ful for  purposes  of  reference.  The  latter  table  is  re- 
printed from  that  which  I  have  given  in  my  "  Intro- 
ductory Lecture  on  Human  and  Comparative  Physio- 
logy/' published  in  1826,  with  only  such  alterations 
as  were  required  to  make  it  correspond  with  the  se- 
cond and  improved  edition  of  Cuvier's  w^ork. 


NOTICE. 


The  series  of  Treatises,  of  which  the  present  is  one,  is  pub- 
lished under  the  followino;  circumstances: 


The  Right  Honourable  and  Reverend  Francis  Henry, 
Earl  of  Bridgewater,  died  in  the  month  of  February,  1829; 
and  by  his  last  Will  and  Testament,  bearing  date  the  25th  of 
February,  1825,  he  directed  certain  Trustees  therein  named  to 
invest  in  the  public  funds  the  sum  of  Eight  thousand  pounds 
sterling;  this  sum,  with  the  accruing  dividends  thereon,  to  be 
held  at  the  disposal  of  the  President,  for  the  time  being,  of  the 
Royal  Society  of  London,  to  be  paid  to  the  person  or  persons  no- 
minated by  him.     The  Testator  farther  directed,  that  the  person 
or  persons  selected  by  the  said  President  should  be  appointed  to 
write,  print,  and  publish  one  thousand  copies  of  a  work  On  the 
Power,    Wisdom,  and  Goodness  of  God,  as  manifested  in  the 
Creation;  illustrating  such  loork  by  all  reasonable  arguments,  as 
for  instance  the  variety  and  formation  of  God's  creatures  in  the 
animal,  vegetable,  and  mineral  kingdoms,-  the  effect  of  digestion, 
and  thereby  of  conversion;  the  construction  of  the  hand  of  man, 
and  an  infinite  variety  of  other  arguments;  as  also  by  discove- 
ries ancient  and  modern,  in  arts,  sciences,  and  the  whole  extent 
of  literature.    He  desired,  moreover,  that  the  profits  arising  from 
the  sale  of  the  works  so  publislied  siiould  be  paid  to  the  authors 
of  the  works. 

The  late  President  of  the  Royal  Society,  Davies  Gilbert,  Esq. 
requested  the  assistance  of  his  Grace  the  Archbishop  of  Canter- 


XIV 


bury  and  of  the  Bishop  of  London,  in  determining  upon  the  best 
mode  of  carrying  into  effect  the  intentions  of  the  Testator.  Act- 
ing with  their  advice,  and  with  the  concurrence  of  a  nobleman 
immediately  connected  with  the  deceased,  Mr.  Davies  Gilbert 
appointed  the  following  eight  gentlemen  to  write  separate  Trea- 
tises on  the  different  branches  of  the  subject  as  here  stated: 

THE  REV.  THOMAS  CHALMERS,  D,  D. 

PROFESSOR  OF  DIVINITY  IN  THE  UNIVERSITY  OF  EDINBURGH. 

ON  THE  POWER,  WISDOM,  AND  GOODNESS  OF  GOD  AS  MANIFESTED 

IN  THE  ADAPTATION  OF  EXTERNAL  NATURE  TO  THE  MORAL 

AND  INTELLECTUAL  CONSTITUTION  OF  MAN. 


JOHN  KIDD,  M.  D.  F.  R.  S. 

REGIDS  PROFESSOR  OF  MEDICINE  IN  THE  UNIVERSITY  OF  OXFORD. 

ON  THE  ADAPTATION  OF  EXTERNAL  NATURE  TO  THE  PHYSICAL 

CONDITION  OF  MAN. 


THE  REV.  WILLIAM  WHEWELL,  M.  A.  F.  R.  S. 

FELLOW  OF  TRINITY  COLLEGE,  CAMBRIDGE. 

ASTRONOMY  AND  GENERAL   PHYSICS  CONSIDERED  WITH 
REFERE.NCE  TO  NATURAL  THEOLOGY. 


SIR  CHARLES  BELL,  K.  G.  H.  F.  R.  S.  L.  &  E. 

THE  HAND  :  ITS  MECHANISM  AND  VITAL  ENDOWMENTS 
AS  EVINCING  DESIGN. 


\ 


PETER  MARK  ROGET,  M.  D. 

FELLOW  OF  AND    SECRETARY  TO  THE  ROYAL  SOCIETY. 

ON  ANIMAL  AND  VEGETABLE  PHYSIOLOGY. 


XV 


THE  REV.  WILLIAM  BUCKLAND,  D.  D.  F.  R.  «. 

CANON  OF  CHRIST  CHURCH,  AND  PROFESSOR  OP  GEOLOGY  IN  THE 
UNIVERSITY  OF  OXFORD. 

ON  GEOLOGY  AND  MINERALOGY. 


THE  REV.  WILLIAM  KIRBY,  M.  A.  F.  R.  S. 

ON  THE  HISTORY,  HABITS,  AND  INSTINCTS  OF  ANIMALS. 


WILLIAM  PROUT,  M.  D.  F.  R.  S. 

CHEMISTRY,  METEOROLOGY,  AND  THE  FUNCTION  OF  DIGESTION, 
CONSIDERED  WITH  REFERENCE  -yO  NATURAL  THEOLOGY. 


His  Royal  Highness  the  Duke  of  Sussex,  President  of  the 
Royal  Society,  having  desired  that  no  unnecessary  delay  should 
take  place  in  the  publication  of  the  above  mentioned  treatises, 
they  will  appear  at  short  intervals,  as  they  are  ready  for  publica- 
tion. 


CO]^TE]\TS 
OF  THE  FIRST  VOLUME. 

INTRODUCTION. 

Chapter  I. — Final  Causes     .  .  .  .  .  .17 

II. — The  Functions  of  Life       ....  39 

PART  I.— THE  MECHANICAL  FUNCTIONS. 

Chapter  I. — Organic  Mechanism       .             .             .             .  .56 

§  1.  Org-anization  in  general  ....  56 

2.  Veg-etable  Org-anization    .             .             .             .  .60 

3.  Development  of  Vegetables  ....  71 

4.  Animal  Organization         .             .             .             .  .79 

5.  Muscular  Power          .  •            ...  97 

Chapter  II. — The  Mechanical  Functions  in  Zoophxtes       .  .     110 

§  1.  General  Observations  .  .  .  .110 

2.  Porifera,  or  Sponges        .....     113 

3.  Polypifera        ......  122 

4.  Infusoria  .  .  .  .  .  .  .136 

5.  Acalepha         ......  142 

6.  Echinodermata     ......     147 

Chapter  III. — Mollusc  a  .  .  .  .  .  .157 

§  1,  Mollusca  in  general  .  .  .  .  .157 

2.  Acephala         ......  159 

3.  Gasteropoda  ......     166 

4.  Structure  and  formation  of  the  Shells  of  Mollusca        .  168 

5.  Ptcropoda  .  .  .  .  .  .185 

6.  Cephalapoda   ......  186 

Chapter  IV. — Articulata     ......     193 

§  1.  Articulated  animals  in  general  .  .  .  193 

2.  Annelida  .  .  .  .  .  .  .194 

3.  Arachnida        ......  202 

4.  Crustacea  ......     204 

Vol.  I.  C 


XVI 11 


CONTENTS. 


Chapter  V. — Insects         ..... 
§  1.  Aptera      ..... 

2.  Insecta  alata    ..... 

3.  Development  of  Insects    .  .  .  , 

4.  Aquatic  Larv?e  .... 

5.  Terrestrial  Larvae  .  .  .  , 

6.  Imago,  or  perfect  Insect 

7.  Aquatic  Insects     .... 

8.  Progressive  motion  of  Insects  on  land 

9.  Flight  of  Insects  .  .  .  . 

Chapter  YI. — Vertebrata  .... 

§  1.  Vertebrated  Animals  in  general    . 

2.  Structure  and  composition  of  the  Osseous  Fabric 

3.  Formation  and  development  of  Bone 

4.  Skeleton  of  the  Vertebrata 


212 
212 
214 
215 

220 
222 
225 
237 
229 
242 

254 
254 
256 
263 
269 


Chapter  VII. — Fishes 


284 


Chapter  VIII. — Reptilia 

§  1.  Terrestrial  Vertebrata  in  general 

2.  Batrachia 

3.  Ophidia    .  .  .  , 

4.  Sauria 

5.  Chelonia 


302 
302 
303 
310 
317 
321 


Chapter  IX. — Mammalia  .... 

§  1.  Mammalia  in  general 

2.  Cetacea  ..... 

3.  Amphibia  .... 

4.  Mammiferous  Quadrupeds  in  general 

5.  Ruminantia  .... 

6.  Solipeda  ..... 

7.  Pachvdermata       .... 

8.  Rodentia  ..... 

9.  Insectivora  .... 

10.  Carnivora         ..... 

11.  Quadrumana         .... 

12.  Man    .  .  .  . 

Chapter  X. — Vertebrata  capable  of  Flyixg 

§  1.  Vertebrata  without  feathers,  formed  for  flying 
2.  Birds        ..... 


330 
330 


oob 

rsrthf 

345 

356 

357 

360 

3( 

364 


362 


367 
369 


376 
376 
382 


LIST  OF  ENGRAVINGS. 


VOLUME  I. 


Fig.  Page 

1  Rotifer  redivivus,  (from  Muller)    ....  58 

2  Vibrio  iritici,  (Bauer)  .            .            .             .            .  .58 

3  Simple  vegetable  cells,  (Slack)      ....  61 

4  Fucus  vesiculosus,  transverse  section,  (De  Candolle)  .  .       til 

5  Ditto,  longitudinal  section,  (id.)       .             .             .             .  61 

6  Compressed  cells  of  vegetables,  (Slack)           .            .  .61 

7  Hexagonal  and  elongated  cells,  (id.)            ...  61 

8  Elongated  cells,  (id.)    .            .            .            .            .  .61 

9  Fibrous  cells,  (id.)  ......  61 

10  Reticulated  cells,  (id.)  .            .            .            .            .  .61 

12  Junction  of  cells  to  form  a  tube      ....  6.5 

13  Beaded  vessels             .            .            .            .            .  .65 

14  Spiral  vessels,  or  Trachese  .....  65 

15  Annular  vessels            .            .            .            .            .  .65 

16  Punctuated  vessels              .....  65 

17  Transitions  of  vessels  from  one  class  to  another            .  .      65 

18  Woody  fibres          ......  65 

19  NervLiresof  a  leaf        .            .            .            .            .  .65 

20  Cells  composing  the  cuticle,  (De  Candolle)            .            .  69 

21  Stomata  magnified,  (Amici)      .             .             .             .  .09 

22  Arrangement  of  stomata  in  cuticle,  (De  Candolle)              .  69 

23  Roots  terminated  by  spongioles,  (id.)    .            .            .  .69 

24  Cells  composing  a  spongiole,  (id.)  ....  69 

25  Animal  cellular  substance        .            .            .            .  .82 

26  Blood  vessel   /......  84 

27  Section  of  blood  vessel,  with  the  valves  open   .            .  .84 

28  Ditto,  with  the  valves  closed           ....  84 

29  Striated  surface  of  the  scale  of  the  Cyprinus  albiirnus,  (Hei- 

singer)              .             .             .  qo 

30  Ditto  of  thePerc«j?wm«a7/s,  (Carus)  .            .            .  .92 

31  Imbricated  arrangement  of  the  scales  of  fishes,  (Ileisinger)  92 

32  Section  of  the  bulbs  of  hair,  magnified              .            .  .93 

33  Quill  of  Porcupine,  (F.  Cuvier)      ....  96 

34  Transverse  section  of  the  same,  (id.)    .            .            .  .96 


XX 


LIST  OF  ENGRAVINGS. 


Fig. 

35  Longitudinal  section  of  the  root  of  ditto,  (id.) 

36  Capsule  of  bulb  of  ditto  laid  open,  (id.) 

37  Muscle  in  a  state  of  relaxation 

38  The  same  muscle  contracted    . 

39  Diagram  illustrating  the  action  of  oblique  muscles 

40  Semi-penniform  muscle 

41  Penniform  muscle  .... 

42  Complex  muscle  .... 

43  Tendon  of  muscle   .  . 

44  Trapezius  muscle         .... 

45  Muscular  structure  of  the  Ear-drum,  (Home) 

46  Orbicular  muscle  of  the  Eye-lids,  (Albinus)      . 

47  Muscular  structure  of  the  Iris,  (Home) 

48  Muscular  fibres  of  a  sucking  disk 

49  Longitudinal  muscular  fibres  of  a  blood  vessel 

50  Transverse  muscular  fibres  of  ditto 

51  Muscular  fibres  of  the  human  stomach,  (Cooper) 

52  Muscular  fibres  of  the  heart,  (id.) 

53  Magnified  view  of  a  Sponge,  (Grant) 

54  Spicula  in  the  texture  of  a  Sponge,  (id.) 

55  Gemmule  of  a  AS|po?2^e,  (id.) 

56  Lobularia.     Alcyonium  pelasgica,  (Deterville) 

57  Detached  polype  of  ditto,  (id.) 

58  Zoanthus,  (Actinia  sociata,)  (Ellis) 
.59  Hydra  viridis,  (Trembley) 

60  Sertularia  pelasgica,  (Deterville) 

61  Tuhipora  musica,  (Ellis)    . 

62  Section  and  polypes  of  ditto,  magnified,  (id.) 

63  Flustra  carbasea,  (id.) 

64  Cells  of  ditto,  magnified,  (id.)  . 

65  Corallium  ruhriim,  (id.)     . 

66  Polypes  of  ditto,  magnified,  (id.) 

67  Section  of  Gorgonia  Briareus,  (id.) 

68  Isis  hippuris,  (id.)        . 

69  Polype  of  Flustra  carbasea,  (Grant) 

70  Tentaculum  of  ditto,  magnified,  (id.)    . 

71  Pennatula  phosphor ea,  (Ellis) 

72  Magnified  view  of  the  polypes  of  ditto,  (id.) 

73  to  76  mode  of  progression  of  the  Hydra  viridis,  (Trembley) 

77  Vorticella  cyathina,  (Muller)  . 

78  Proteus  dijjiuens,  (id.) 

79  Volwx  globator,  (id.) 

80  Brachionus  urceolaris,  (id.) 


Page 
96 
96 
101 
101 
101 
101 
101 
101 
101 
101 
105 
105 
105 
105 
106 
106 
106 
106 
114 
114 
114 
122 
122 
122 
122 
124 
125 
125 
125 
,  125 
125 
.  125 
125 
.  125 
129 
.  129 
131 
.  131 
133 
.  136 
139 
.  139 
140 


LIST  OP  ENGRAVINGS.  XXi 

Fig. 

81  Medusa  Pulmo,  (Maori)         .  ,  .  .  .     rS 

82  Beroe  ovatus,  (Bruguiere)            •            •            •            .  144 
SZ  Beroe  pileus,  {vi\,) j^ 

84  Velella  limbosa,  (Guerin)  ....  144 

85  Phijsalia  atlantica,  (id.)         .  .  .  ^  .144 

86  Actinia  rvfa,  (original)    .....  \/^a 

87  Ditto  expanded,  (original)       ....  145 

88  Asterias  serrulata,  (Bruguiere)    ....  147 

89  Asterias  regularis,  (id.)  .  .       •     .  ^  -i/Vl 

90  Echinus  Ananchites  ovata,  (id.)  ....  147 

91  Clypeaster  rosaceus,  (id.)       .....     147 

92  Ophiura  lacertosa,  (id.)    .....  147 

93  Euryale  muricatum,  (id.)       .  .  .  ^  24^ 

94  Pentacrinus  europa^us,  (Thomson)  .  .  .147 

95  Ambulacra,  and  feet  o^  Asterias,  viewed  from  the  under  side, 

(Reaumur) '       148 

96  Ditto,  viewed  from  the  upper  side,  (id.)         .  .  .148 

97  Vesicles  appended  to  the  feet  of  the  Asterias       .  .148 

98  Polygonal  pieces  composing  the  test  of  the  Echinus  .  .     150 

99  Structure  of  a  detached  piece  of  ditto        .  .  .      '     15() 

100  Spine  of  the  Cidaris,  (Carus)  .  .  .       *     .     150 

101  Shell  o^-Unio  batava,  (Goldfuss)  .  .  .  ,      '     j^g 

102  Adductor  muscle  of  Oyster,  (Hunterian  Museum)     .  .     I'eo 

103  Shell  orPholas  Candida,  with  abductor  muscle,  (Osier)    .  161 

104  Foot  of  Cardium  edule,  (Reaumur)    .  .  .  .162 

105  Planorbus  cornutus  (Cuvier)       ...  *     166 

106  Magnified  view  of  the  striae  on  the  surface  of  Mother'  of 

Pearl,  (Herschel)        ••  ...  169 

107  Directions  of  the  fibres  in  the  component  strata  of  shells        .  170 

108  Shell  of  Ac/i«fm«ze6r«,  (DeBlainville)  .  .  I'ya 

109  Longitudinal  section  of  ditto,  (id.)      .  .  ,       '  j-^g 

110  Shell  ofPterocerus  scorpio,  at  an  early  stage  of  growth,  (id.)  '  178 

111  Shell  of  the  same  when  completely  formed,  (id.)       .  .  173 

112  Shell  of  Ci/proia  exanthema  at  an  early  period  of  growth,  (id.)  '  178 

113  Shell  of  the  same  animal,  when  completed,  (id.)        .  179 

114  Transverse  section  of  the  shell  of  the  CyprcBa  exanthema, ' 

(Hunterian  Museum)        .  i-o 

115  Shell  of  CoMMs      .  ..'.**  jQj 

110  Longitudinal  section  of  the  same,  (original)  .  .  .181 

117  Transverse  section  of  the  same,  (Brug°jiere)         .      *      .      *     181 

118  Inner  surface  of  the  Epiphragma  of  the  Helix  pomatia,  (De 

^l^'"vil^e)       • 183 


xxu 


LIST  OF  ENGRAVINGS. 


Fig. 

119  Outer  surface  of  the  same,  (id.) 

120  Clio  borealis,  (Cuvier) 

121  Sepia  loligo,  (De  Blainville) 

122  Suckers  of  the  same,  (id.) 

123  Suckers  of  the  Octopus,  (original)     . 

124  Shell  of  Spirilla  australis,  (De  Blainville) 

125  Longitudinal  section  of  the  same,  (id.) 

126  Shell  of  JSautilus  pompilius  (id.) 

127  Longitudinal  section  of  the  same,  (id.) 

128  Pontobdella  muricata,  (Bruguiere) 

129  Nereis,  (id.)   . 

130  Erpobdella  vulgaris  (Lam.)  Hirudo  hyalina 

131  Diagram  illustrating  the  rings  and  muscles  of  Annelida, 

(original)         .  .  .  , 

132  Gordius  aquaticus     .... 

133  Serpula  opercularia 

134  Terebella  conchilega,  (De  Blainville) 

135  Arenicola  piscatorum,  or  Lumbricus  marinus 

136  Aranea  diadema,  (Rcesel) 

137  Divisions  of  the  limb  of  a  Crustaceous  animal 

138  Mandible  and  palpus  of  My  sis  Fabricii,  (Bruguiere) 

139  to  141  Feet-jaws  belonging  to  the  first,  second,  and  third 

pairs,  (id.) 

142  True  foot,  belonging  to  the  first  pair,  (id.) 

143  Julus  terrestris  ..... 

144  Muscles  of  the  trunk  of  the  Melolontha  vulgaris,  (Straus 

Durckheim)  ..... 

145  Eggs  of  Bombyx  mori       ..... 

146  Larva  of  the  same       ..... 

147  Pupa  of  the  same  ...... 

148  Imago  of  the  same      ..... 
148*  A  Caterpillar  of  the  Phalena  striaria,  (Hubner) 

B  The  same  in  a  rigid  position,  (Lyonet) 

149  Calosoma  Sycophanta,  (Kirby  and  Spence) 

150  Analysis  of  skeleton  of  the  same,  (Carus) 

151  Hind  vievi?  of  the  segment  of  the  head  in  the  same,  (id.)  . 

152  Suckers  on  the  foot  of  the  Musca  vomitoria,  expanded ;  mag- 

nified view,  (Bauer)    ..... 

153  Cushions  on  the  foot  of  the  Cimbex  lutea,  magnified,  (id.) 

154  Suckers  on  the  under  side  of  the  foot  of  a  male  Dytiscus 

marginalis,  (id.)  .  .  .  .  . 

155  Cushions  and  sucker  of  the  Acridium  biguttulum,  Latr.  (id.) 


Page 

183 
186 

187 
187 
187 
191 
191 
191 
191 
195 
195 
195 

195 
198 
198 
198 
198 
202 
205 
205 

205 
205 
213 

214 
217 
217 
217 
217 
224 
224 
227 
228 
228 

235 
235 

235 
235 


LIST  OF  ENGRAVINGS.  xxiii 

I^'g-  Page 

156  Dytiscus  marginalis,  upper  side,  (Roesel)      .            .            .  2:37 

157  Lower  side  of  the  same  insect,  (id.)          .            .            .  287 

158  Notonecta  glauca,  (Rccsel)    .            .            .            .            .  238 
158*  Fore  leg  of  Gryllotalpa,  (Kidd)                .            .            .  242 

159  Wing  of  Gryllus  nasutus.     Orthoptera          .            .            .  246 
1(S0  Wmg  0^  Lihellula  grandis.     Neuroptera              .            .  246 

161  Wing  of  Ichneumon  persuasorius.     Ilymenoptera    .            .  246 

162  Wing  of  Tipula  oleracea.     Diptera          .            .            .  246 

163  Sting  of  Antliophora  retusa,  (original)           .            .            .  248 

164  Separate  scales  of  the  wing  oT  Hesperia  Sloanus,  (original)  250 

165  Arrangement  of  the  scales  in  the  wing  of  the  same    .            .  250 

172  Longitudinal  section  of  the  thigli-bone  to  show  the  cancel- 

lated structure,  (Cheselden)          ....  262 

173  Longitudinal  section  of  the  humerus,  (id.)            .            .  262 

174  Ossification  of  the  parietal  bone,  (id.)             .            .            .  265 

175  Early  stage  of  ossification  of  the  bones  of  the  skull,  (Cloquet)  265 

176  The  same  in  the  adult,  showing  the  sutures  .            .            .  265 

177  Dorsal  vertebra,  human     .....  271 

178  Junction  of  vertebrse  forming  the  spinal  column          .             .  271 

179  Longitudinal  section  of  the  same,  showing  the  spinal  canal  271 

180  Elements  of  structure  of  a  vertebra,  (Cams)  .            .            .  274 

181  Skeleton  of  Hog,  (Pander  and  D'Alton)   .  .  .279 

182  Sternum,  clavicle,  and  scapula ;  human          .            .            .  279 

184  Skeleton  of  Cyprinus  carpio,  (Bonnaterre)          .            .  286 

185  Diagram  illustrating  the  progressive  motion  of  Fishes            .  287 

186  Front  view  of  the  vertebra  of  a  Cod,  (Gadus  morrhua)   .        '  288 

187  Side  view  of  the  same            .....  288 

188  Vertical  and  longitudinal  section  of  a  part  of  the  spinal  co- 

lumn in  the  same              .....  288 

189  A  similar  section,  showing  the  gradation  of  structure       .  288 

190  Similar  section  in  the  Squalus  centrina,  (Carus)        .             .  288 

191  Bones  0^  the  shoulder  of  the  Lophius  piscatorius,  (id.)     .  293 

192  Pectoral  fin  of  the  Raia  davata,  (id.)            .            .            .  293 

193  Belt  of  bones  of  the  shoulder  of  a  Ray,  (id.)          .            .  294 

194  Muscular  system  of  Cyprinus  alburnus,  (id.)            .            .  295 

195  Air  bladder  of  Cyprinus  carpio,  (Blasius)            .            .  298 

196  Eggs  of  the  Frog        .  .       ,     .  .  .  ,303 

197  Side  view  of  Tadpole  magnified,  (Rusconi)          .            .  303 

198  Upper  view  of  the  same,  (id.)            ....  303 

199  Adult  Frog '  303 

200  Skeleton  of  Frog,  (Cheselden)            ....  306 

201  Skeleton  of  the  Viper       .....  310 


Xxiv  LIST  OF  ENGRAVINGS. 

Fig.  I'^g^ 

202  Ribs  and  spine  of  Boa  cons/ncfor,  (Home)    .  .  .    312 

203  Bones  of  the  foot  of  the  same,  (Mayer)     .  .  .311 

204  Muscles  moving  the  claw  of  the  same,  (id.)  .  .  .     311 

205  Rudimental  bones  of  the  foot  of  the  Tortryx  scytale,  (id.)  311 

206  oHhe  Tortrix  corallinus,  (\di.)  .             .            •     311 

207  of  the  Anguis  fragilis,  (id.)  .             .             .          311 

208  of  the  Amphisbcena  alba,  (id.)  .             .             •     311 

209  of  the  Coluber  pullutatus,  (\A.)  .             .            .           311 

210  Chalcides  pentadacttjlus,  (Bonnaterre)  .  .  •     311 

211  Under  surface  of  the  foot  of  the  Lacerta  gecko,  magnified 

four  times,  (Bauer)  .....     319 

212  Side  view  of  a  longitudinal  section  of  the  same,  (id.)        .  319 

213  Skeleton  of  the  Tortoise,  (Carus)       .  .  .  .322 

214  Section  of  the  thigh  bone  of  the  same,  (id.)  .  .  322 

215  Hind  view  of  skull  of  Testudo  mydas,  (id.)    .  .  .     325 

216  Bones  sustaining  the  fin  of  the  Delphinus  phoccena,  (Pander 

and  D' Alton) 336 

217  Fore  part  of  the  Skeleton  of  an  Ox  with  the  Ligamentum 

nucli(B,  (original)  .....     346 

218  Skeleton  of  the  Stag,  (Cheselden)  .  .  .350 
218*  A.  Longitudinal  section  of  the  horn  of  an  Ox,  (original)        .     355 

B.  Ditto  of  an  Antelope,  (original)  .  .  .  355 

c.  Extremity  of  the  same,  (original)  .  .  .     355 

219  Subcutaneous  muscles  of  the  Hedge-hog,  relaxed,  (Carus)  364 

220  The  same  muscles  contracted,  and  drawn  over  the  body, 

(Cuvier)   .  .  .  .  .  •  .364 

221  Skeleton  of  the  Lion,  (Pander  and  D' Alton)         .  .  365 

222  Skeleton  of  Drflco  ?;oZa?zs,  (Tiedemann)         .  .  .     379 

223  Skeleton  of  Vespertilio  Molossus,  (Temmink)     .  .  380 

224  Skeleton  of  the  Swan,  (Cheselden)    ....     385 

225  Lateral  section  of  the  cervical  vertebra  of  the  Ostrich,  (ori- 

ginal) ......  388 

226  Fibrils  of  the  vane  of  a  feather,  magnified,  (original)  .     393 

227  Edges  of  the  fibres,  magnified,  (original)  .  .  .  393 

228  Feather,  showing  its  structure,  (F.  Cuvier)   .   "         .  .     396 

229  Capsule,  or  Matrix  of  the  feather,  (id.)     .  .  .396 
•  230  View  of  the  parts  enclosed  in  the  Capsule,  when  laid  open, 

(id.) 396 

231  Section  of  the  stem,  while  growing,  exhibiting  the  series  of 

conical  membranes,  (id.)         ....  396 

233  Extensor  muscles  of  the  foot  and  toes  of  a  bird,  (Borelli)       .  405 

234  Position  of  a  bird  in  roosting,  (id.)  .  .  .  405 


LIST  OF  ENGRAVINGS.  XXV 


VOLUME  II. 

Fig.  Page 

239  Cyclosis,  or  partial  circulation  in  the  cells  of  the  Caulinia 

fragilis,  magnified,  (Amici)  .  .  .  .42 

240  The  same  in  the  jointed  hair  of  the  Tradcscantia  virginica, 

(Slack)      .......  42 

241  Section  of  the  Hydra  vividis,  magnified,  (Trembley)        .  58 

242  Hydra  vividis  seizing  a  worm,  (id.)  .  .  .  .59 

243  The  same  after  swallowing-  a  minnow,  (id.)  .  .59 

244  A  Hydra  which  has  swallowed  another  of  its  own  species,  (id.)  59 

245  Compound  Hydra,  with  seven  heads,  (id.)             .             .  59 

246  Veretilla  lutea,  showing  the  communicating  vessels  of  the  Po- 

lypes, (Quoy  et  Gaimard)        ....  64 

247  Nutrient  vessels  of  the  TcBiiia  solium  (Chiaje)          .            .  64 

248  T(Enia  globosa,  or  Hydatid  of  the  Hog,  (Goeze)               .  64 

249  Horizontal  section  of  the  Rhizostoma  Cuvieri,  Peron,  (Ey- 

senhardt)         .            .            .            .            .            .  67 

250  Geronia  HexaphyUa,I*cron,  Medusa  proboscidalis,  (Forskal)  67 

251  Vascular  net-work  in  margin  of  the  disk  of  the  Rhizostoma 

Cuvieri,  (Eysenhardt)              ....  67 

252  Vertical  section  of  the  Rhizostoma  Cuvieri,  (id.)      .            .  68 

253  Transverse  section  of  one  of  the  arms  of  the  same,  (id.)  .  68 

254  Transverse  section  of  the  extremity  of  a  tentaculum  of  the 

same,  (id.)       ......  68 

255  Leucophra  patula,  highly  magnified,  (Ehrenberg)     .             .  73 

256  Alimentary  canal  and  cffica  of  the  same,  viewed  separately, 

(id-) 73 

257  Vertical  section  of  the  Acrmzrt  conrtcea,  (Spix)    .            .  75 

258  Digestive  organs  of  the  Ai-^enas,  (Tiedemann)          .             .  76 

259  Siomsichs  oHhe  Na  is  vermicular  is,  (RcBsel)        .             .  77 

260  Stomachs  of  the //«Vw(/o  ?nefZicmaZi5,  (original)          .            .  78 

261  Mouth  of  the  same,  showing  the  three  semicircular  teeth, 

(original)               ••-...  78 

262  Tooth  of  the  same,  detached,  (original)    ...  78 

263  Glassopora  tuberculata;  Hirudo  complanata,  Lin.  (Johnson)  78 

264  The  same  seen  from  the  under  side,  showing  the  digestive 

organs,  (id.) 79 

265  Diagram  showing  the  arrangement  and  connexions  of  the  or- 

gans of  the  vital  functions  in  Vertebrata,  (original)     .  81 
Vol.  I.                                 D 


XXVI  LIST  OF  ENGRAVINGS. 

Fig.  Page 

266  Spiral  probosces  of  Papilio  urticcc,  (Griffith)              .            .  87 

267  Trophi  of  Locusta  viridissima,  (Goldfuss)       ,,    .             .  92 

268  Filaments  composing  the  rostrum,  or  proboscis,  of  the  Cimex 

nigricornis,  (Savigny)      .  .  .  .  .94 

269  Sheath  of  the  proboscis  of  the  same  insect,  (id.)     .            .  94 

270  Toothed  cartilage  of  tlie  f/eZia;_po7n«fra,  (Cuvier)      .             .  95 

271  Mechanism  for  projecting  and  retracting  the  tongue  of  the 

Woodpecker,  (original)            ....  99 

272  Laminse  of  Whalebone  descending  from  the  palate  of  the  Ba- 

IcBna  mysticetus,  (Bonnaterre)      ....  102 

273  Teeth  of  the  Delphinus  phoccena,  (Cloquet)          .             .  106 

274  Skull  of  Tiger,  (Cuvier) 108 

275  Skull  of  Antelope,  (Pander  and  D'Alton)  .             .             .  109 

276  Skull  of  Rat,  (id.)      .  .  .  .  .  .110 

277  Longitudinal  section  of  simple  tooth,  (Rousseau)               .  Ill 

278  Surface  of  the  grinding  tooth  of  a  Horse,  (Home)       .            .  Ill 

279  Surface  of  the  grinding  tooth  of  a  Sheep,  (id.)       .            .  Ill 

280  Lonoitudinal  section  of  the  incisor  tooth  of  the  Rodentia        .  Ill 

281  Vertical  section  of  the  grinding  tooth  of  the  Elephant,  (Home)  114 

282  Grinding  tooth  of  the  African  Elephant,  (id.)         .             .  114 

283  Grinding  tooth  of  the  Asiatic  Elephant,  (id.)  .             .             .  114 

284  Succession  of  teeth  in  the  Crocodile,  (Carus)        .             .  120 

285  Venomous  fang  of  the  Coluber  naia,  (Smith)              .             .  121 

286  Transverse  section  of  the  same,  (id.)         .             .             .  121 

287  The  same  tooth,  at  an  earlier  period  of  growth,  (id.)  .             .  121 

288  The  same,  still  less  advanced  in  its  growth,  (id.)              .  121 

289  Base  of  the  former,  (id.)          .....  121 

290  Base  of  the  latter,  (id.)     .....  121 

291  Transverse  section  of  the  young  fang,  about  its  middle,  (id.)  121 

292  A  section,  similar  to  the  last,  of  another  species  of  serpent,  (id.)  121 

293  Squalus  pristus.    b.  Under  side  of  its  snout,  (Latham)           .  122 

294  Interior  of  the  Stomach  of  a  Lobster,  (original)     .             .  123 

295  Gastric  teeth  of  BuUcea  aperta,  (Cuvier)        .             .             .  123 

298  Gizzard  of  the  Swan,  (Home)       ....  124 

299  Crop  and  gizzard  of  the  Parrot,  (id.)    v          .             .             .  131 

300  Crop  of  the  Pigeon,  (id.)  .  .  .  .  .131 

301  Human  stomach,  (id.)              .....  133 

302  Interior  of  the  stomach  of  the  African  Ostrich,  (id.)         .  135 

303  Gastric  glands  of  the  same,  (id.)         ....  135 

304  Gastric  glands  of  the  American  Ostrich,  (id.)       .             .  135 

305  Longitudinal  section  of  the  gastric  glands  of  the  Beaver,  (id.)  135 

306  Stomach  of  Dormouse,  (id.)    .....  139 


LIST  OP  ENGRAVINGS.  XXVii 

^07  Stomach  of  Hi/rax  capensis,  (Cuvier)      .            .  .          ^39 

308  Stomach  of  Porcupine,  (id.)  ....  139 

309  Stomach  of  iTflw^^t/roo,  (id.)          .            .            .  109 
'SIO  Stomach  of  Delphinus  phoccBna,  (\^l.)             .             .  ^     239 

311  Cardiac  valve  of  the  Horse,  (Gurlt)  .  .  .      *     140 

312  The  four  stomachs  of  a  Sheep,  (Carus)  .  .  .141 

313  Ini>er  surface  of  the  honey-comb  stomach,  (Home)  .  141 

314  Inner  surface  of  the  many-plies  stomach  of  an  Ox,  (id.)  .     141 

315  Interior  cellular  surface  of  the  second  stomach  of  the  Camel 

<^'^'^     •  •  •  .  .  .  .      '  141 

316  Spiral  valve  in  the  intestine  of  the  Shark,  (Blasius)  .  .  149 

317  Digestive  organs  of  the  Mantis  religiosa,  (Marcel  de  Serres)  153 

^1^  Melolontha  vulgaris,  (Leon  Dufour)        .  .  154 

^1^  Cicindela  campestris,  (id.)     .  .  ,  J54 

320  Portion  of  a  hepatic  vessel  of  the  Melolontha,  highly  magni- 

fied, (Straus  Durckheim)         ....  155 

321  Alimentary  canal  of  the  .4cn^«cj9ferff,  (original)      .  .  155 

322  Interior  of  the  gizzard  of  the  same,  magnified,  (oriainal)  .  155 

323  Row  of  large  teeth  in  the  same,  still  more  magnified,^  (original)  155 

324  Profile  of  one  of  those  teeth,  still  more  highly  magnified"  (ori- 

g^"al) j^ 

325  Base  of  the  same  tooth,  seen  from  below,  (original)  .  155 

326  Alimentary  canal  of  the  Larva  of  the  Spkinx°ligustri,  (orio-i- 

nal) °.     157 

P"^  ■ of  the  Pupa  of  the  same,  (original)           .  .157 

328  of  the  Imagoof  the  same,  (original)  .             .  157 

329  of  the  Patella,  (Cuvier)   .            .            .      '  .      *     159 

330  Stomachs  of  the  PZeMroirancAiisPeromf,  (id.)  .  .     159 

331  Pyloric  appendices  in  the  Salmon,  (id.)    .  .  .160 

333  Detached   Dorsal    vessel  of   Melolontha   vulgaris,   (Straus 

Durckheim)  .  .  .  _  _  .172 

334  The  same,  with  its  ligamentous  and  muscular  attachments* 

O^O '  172 

335  Side  view  of  the  anterior  extremity  of  the  same  vessel,  (id.)  172 

336  Section  of  the  dorsal  vessel,  to  show  its  valves,  (id.)  .            .  170 

337  Circulation  iit  the  antenna  of  the  Semblis  viridis,  (Carus)     *  17.5 

338  Course  of  circulation  in  the  same  insect,  (id.)            .  175 

339  Dorsal  vessel  of  the  Caterpillar  of  the  Sphinx  ligustri,  side 

view,  (original)  .  ^  j^y 

340  The  same  in  the  Chrysalis,  (original)  .            .  '     .  177 

341  The  same  in  the  Moth,  (original)  .            .      *            *  177 

342  The  same  viewed  from  above,  (original)  .            .  *     .  177 


XXVlll 


LIST  OP  ENGRAVINGS. 


Fig. 

343  Magnified  lateral  view  of  the  anterior  extremity  of  the  dorsal 

vessel,  (original)  .  •  .  .  . 

344  Magnified  dorsal  view  of  the  same,  (original) 

345  Structure  of  the  valves  of  the  dorsal  vessel,  (original,)      . 

346  Heart  and  vessels  of  the  Amnea  dotnestica,  (Treviranus) 
346*  Circulation  in  the  Planaria  nigra,  (Duges) 

347  Course  of  circulation  in  the  Erpobdella  vulgaris,  (Morren)  . 

348  Vessels  in  abdominal  surface  of  the  same,  (id.)     . 

349  Vascular  dilatations,  or  hearts  of  the  Lumhricus  terrestris, 

(Morren)  .  .  .  .  • 

350  Cavities  and  great  vessels  of  the  Heart,    . 

351  The  Heart  laid  open  to  show  its  Valves, 

352  Plan  of  simple  circulation, 
'  353  Plan  of  double  circulation, 

354  Branchial  circulation  in  Maia  Squinado,  (Audouin) 

355  Organs  of  circulation  in  the  Loligo  sagittata,  (id.) 

356  Plan  of  circulation  in  Fishes, 

357  Plan  of  circulation  in  Batrachia, 

359  Plan  of  double  or  warm-blooded  circulation, 

360  Heart  of  the  Diigong,  (Home) 

365  Valves  of  the  Veins,  (Cloquet)      . 

366  Heart,  branchial  artery  and  gills  of  a  fish,  (Blasius) 

367  Branchial  apertures  in  the  Squalus  glaucus,  (Bonnaterre) 

368  Branchial  apertures  in  the  Petromyzon  marinus,  (id.) 

369  Internal  structure  of  the  branchias  of  the  same,  (Home) 

370  Stigmata  in  the  abdominal  surface  of  the  Dytiscus  margina- 

lis,  (Leon  Dufour)  .  .  .  .  • 

371  Stigmata  of  the  Ceramhyx  heros,  (Fab.)  magnified,  (id.) 

372  Longitudinal  trachese  of  Carabus  auratus,  (id.)     . 

373  Air  vesicles  and  trachea?  of  the  Scolia  hortorum,  (Fab.)  high- 

ly magnified,  (id.)     ....•• 

374  Respiratory  apparatus  of  the  Scorpio  eiiropcBus,  (Treviranus) 

375  Internal  structure  of  the  lungs  of  the  Turtle,  (Bojanus)     . 

377  Air  cells  of  the  0-s/ric/i,  (Parisian  Academicians) 

378  Lymphatic  Absorbents,      .  ■•  ... 

379  Passage  of  Nerves  through  a  ganglion, 

380  Plexus  of  Nerves,  ..... 

381  Varieties  of  forms  of  antennae  of  Insects,  (Goldfuss)   . 

382  Vertical  and  longitudinal  section  of  the  right  nostril  in  man, 

383  Vertical  transverse  section  of  the  same,    . 

384  Transverse  section  of  the  nostril  of  a  Sheep,  (Harwood) 

385  Turbinated  bones  of  the  »SeflZ,  (id.) 


Page 

177 
177 
177 
180 
181 
183 
183 

184 

187 

188 

189 

191 

193 

195 

196 

197 

199 

200 

206 

216 

216 

216 

216 

222 
222 
222 

222 
225 
229 
233 
250 
255 
255 
272 
283 
284 
285 
285 


LIST  OP  ENGRAVINGS. 


XXIX 


^'^-  Page 

386  Turbinated  bones  of  the  T?<rA:fy,  (id.)  -  .  .  287 

387  Nerves  distributed  to  the  bill  of  the  Duck,  (id.)  -  -  288 

388  Nasal  cavities  of  the  Pero«/2<i;m/27i5,  (Cuvier)         -  .    291 

389  Nasal  cavity  of  the  Raia  balls  or  Skate,  (Harwood)  -  291 

390  Human  ear,  (Cloquet)  -  -  _  _  _  299 

391  Posterior  surface  of  the  cavity  of  tlie  tympanum,  (id.)      -  301 

392  Ossicula  auditus,  or  small  bones  of  the  tympanum,    -  -  301 

393  The  position  of  the  latter  in  the  tympanum,  -  -  301 

394  Magnified  view  oT  the  labyrinth  detached  from  the  surround- 

ing parts,  (Breschet^         -  -  -  .  .  303 

395  Interior  structure  of  the  labyrinth,  (id.)     -  -  -  304 

396  Membranous  labyrinth,  witli  its  nerves,  (id.)  -  .  304 

397  Cretaceous  bodies  in  the  labyrinth  of  the  Dog,  (id.)  -  304 

398  Ditto  in  that  of  the //are,  (id.)  -  -  -  -  304 

399  Organ  of  hearing  in  the  Lobster,  (Cams)  -  -  308 

400  Groove  in  the  sac  of  the  former,  (id.)  .  _  .  308 

401  Organ  of  hearing  in  the  Astacusfliiviatilis,  (id.)  -  308 

402  Interior  view  of  the  same,  (id.)  -  -  .  .  308 

403  Membranous  labyrinth  of  the  i«oyiiM5;)«scG?orms,  (id.)     -  310 

404  Organ  of  hearing  in  the  Frog,  (Bell)  -  -  -  311 

405  Ear  of  the  Turkey,  (Carus)  -  -  -  .  31X 

406  Diagram  illustrating  one  mode  of  obtaining  images  of  objects, 

(original)  -  -  -  -  _  -  319 

407  Simple  Camera  Obscura  -  -  -  -  -  320 

408  Law  of  the  refraction  of  a  ray  of  light,  ...  321 

409  Convergence  of  rays  to  a  fijcus,    -  -  -  _  322 

410  Convergence  by  a  double  conve.x  lens,  -  -  _  324 

411  Spherical  Aberration,       -  -  -  .  _  324 

412  Variations  of  focal  distance  consequent  upon  variations  of  di- 

vergence of  the  incident  rays,       -  -  -  _  325 

415  Horizontal  section  of  right  human  eye,  magnified,  (Home)  326 

416  Straight  and  oblique  muscles  of  the  eye-ball,        -  -  328 

417  Lacrymal  apparatus,  ----..  330 

418  Eye  of //eZix;jowaa'«,  (Muller)  -  -  -  .  339 

419  Stemmata  of  Caterpillar,  (Marcel  de  Serres)  -  -  342 

420  Eye  of  the -Scorpio  fw^icnm,  (Muller)      -  -  .  340 

421  Conglomerate  eyes  of  Julus  terrestris,  (Kirby  and  Spencc)  342 

422  External  magnified  view  of  the  compound  eye  of  the  Melolon- 

tha  vulgaris,  (Straus  Durckhcim)  -  -  -  344 

423  Ditto  of  that  of  a  P/ia?cn«,  -  -  -  -  ;M4 

424  Section  of  the  compound  eye  of  the  Libellula  vulgata,  magni- 

fied, (Duges)         --....  344 


XXX  LIST  OF  ENGRAVINGS. 

Fig.  Page 

425  Highly  magnified  view  of  the  outer  margin  of  the  preceding 

section,  (id.)   ------  344 

426  Portion  of  the  section  of  the  eye  of  the  Melolontha  vulgaris, 

(xMuller)               -            -            -            -            -  -    344 

427  Portion  of  the  section  of  tl)e  eye  of  the  Libellula,  (Dnges)  344 

428  Portion  of  the  section  of  the  eye  of  the  Melolontha  vulgaris, 

(Straus  Durckheim)   -----  344 

430  Interior  of  the  eye  of  the  Percaj^iii;?V/H//s,  (Cuvier)  -    349 

431  Fibres  of  the  crystalline  lens  of  the  Cod,  (Brewster)        -  350 

432  Denticulated  structure  of  these  fibres,  (id.)    -            .  -     350 

433  Section  of  the  eye  of  the  Goose,  (Home)               -            -  353 

434  Nictitating  membrane  of  a  Bird,  (Petit)          -            -  -    353 

435  Muscles  of  the  nictitating  membrane,  (id.)            -            -  353 

438  Talilrus,  (Latreille)              -            -            -            -  -    381 

439  Nervous  system  of  the  Talitrus,  (Audouin)          -            -  382 

440  Nervous  system  of  C^/mof/ioa,  Fab,,  (id.)        -            -  -     382 

441  Nervous  system  of  the  il/aia  s^wma^^o,  (id.)         -            -  383 

442  Nervous  system  of  the  Larva  of  the  Sphinx  ligustri,  (New- 

port)           386 

443  Ditto  of  the  Chrysalis  of  the  same,  (id.)   -            -            -  386 

444  Ditto  of  the  Imago  of  the  same,  (id.)              -            -  -    386 

445  Nervous  system  of  the  Asterias,  (Tiedemann)      -            -  387 

446  Ditto  of  the  jl^Z?/sm,  (Cuvier)            -            -            -  -    387 

447  of  the  Patella,  (id.)          .            ...  387 

448  oHhQ  Sepia  Octopus,  (\A.)    -            -            -  -    387 

449  Brain  and  spinal  marrow  of  the  Cqlumba  turtur,  (id.)      -  389 

450  Transverse  section  of  the  spinal  marrow  of  the  Cyprinus  car- 

pio,          -            -            -            -            -            -  -    389 

451  Brain  and  spinal  marrow  of  the  Trigla  lyra,  (Arsaky)     -  389 

452  Brain  of  the  MurcBua  conger,  (Serres)          -            ,  -    389 

453  Percafuviatilis,  (Cuvier)            .            -            _  389 

454 Testudo  mydas,  (Carus)       -            -            -  -    389 

455  Crocodile,  (id.)     -            -            -            -            -  389 

456  Lion,  (Serres)           -            .            -            _  .    339 

457  Lateral  view  of  the  brain  of  the  Perch,  (Cuvier)  -            -  389 

458  of  the  Testudo  mydas,  (Carus)          -            -  _    389 

459  of  a  section  of  the  brain  of  the  Dove,  (id.)            -  389 

460  of  the  Lion,  -            -            -            -            -  -    389 

461  Vertical  section  of  the  human  brain,  (Monro)      -            -  393 

462  Progressive  changes  in  the  Monas     -            -            -  -    410 

463  Vorticella 410 


OUTIilNE 


OF 


CUVIER'S  CLASSIFICATION  OF  ANIMALS, 

WITH 

EXAMPLES  OF  AI^IMALS  BELONGING  TO  EACH  DIVISION. 


Blmana 

Quadrumana  - 

Cheiroptera  - 

Jnsectivora  - 

Plantigrada  - 

Digitig-rada  - 
Amphibia 

Marsupialia  - 
Rodentia 

Edentata 

Pachydermata 

Ruminantia   - 

Cetacea 

Accipitres     - 

Passeres 

Scansores 

Gallinse 

Grallce 

Palmipedes  - 

Chelonia 
Sauria 
Ophidia 
Batracliia 

Acanthopteryg"ii 
Malacopteiygii 


L  VERTEBRATA. 

1.  Mammalia. 
Man. 

Monkey^  Ape,  Lemur. 

Bat,  Colugo. 

Hedge-hog,  Shrew,  Mole. 

Bear,  Badger,  Glutton. 

Dog,  Lion,  Cat,  3Iarii7i,  Weasel,  Otter. 

Seal,  Walrus. 

Opossum,  Kanguroo,  Wombat. 

Beaver,  Rat,  Squin-el,  Porcupine,  Hare. 
<  Sloth,  Armadillo,  Ant-eater,  Pangolin,   Orni- 
\      thorhyncus. 

Elephant,  Hog,  Rhinoceros,  Tapir,  Horse. 
(  Camel,  Musk,  Deer,   Giraffe,  Antelope,   Goat, 
\      Sheep,  Ox. 

Dolphin,  TVIiale. 

2.  AvES. 

Vulture,  Eagle,  Owl. 

Thrush,  Swallow,  Lark,  Crow,  Sparrow,  Wren. 

Woodpecker,  Cuckoo,  Toucan,  Parrot. 
Peacock,  Pheasant,  Grous,  Pigeon. 
Plover,  Stork,  Snipe,  Jbis,  Flamingo. 
Auk,  Grebe,  Gull,  Pelican,  Swan,  Duck. 

3.  Reptilia. 

Tortoise,  Turtle,  Emys. 

Crocodile,  Lizard,  Gecko,  Chameleon. 

Serpents,  Boa,  Viper. 

Frog,  Salamander,  Newt,  Proteus,  Siren. 

4.  Pisces.      ' 
Perch,  Mackerel,  Swoi'd-fish,  Mullet. 
S  Salmon,  Herring,   Pike,   Carp,   Silurus,  Cod, 
i      Sole,  Re  mora,  Eel. 


XXXll 


CLASSIFICATION  OF  ANIMALS. 


Lophobranchi 

Plectognathi 

Chondropterygii 


1.  Cephalopoda 

2.  Pteropoda 

3.  Gasteropoda 

4.  Acephala 

5.  Brachiopoda 

6.  Cirrhopoda 


Tubicola     - 
Dorsibranchia 
Abranchia  - 

1.  Malacostraca. 
Decapoda 
Stomapoda 
Amphipoda    ■ 
Lccmodipoda  ■ 
Isopoda  - 

2.  Entomostraca 


Pulmonalia 
Trachealia  - 


Aptera 
Coleoptera  - 
Orthoptera  - 
Hemiptera  - 
Neuroptera 
Hymenoptera 
Lepidoptera 
Rhipiplera  - 
Diptera 

1.  Echinodermata 

2.  Entozoa 

3.  Acalephae 

4.  Polypi  - 

5.  Infusoria 


Pike-Jish,  Pegasus. 
Sun-fish,  Trunk-fish. 
Lamprey^  Shark,  Bay,  Sturgeon. 

II.  MOLLUSCA. 

Cuttlefish,  Calamary,  Nautilus. 

Clio,  Hyalaea. 

Slug,  Snail,  Limpet,  Whelk. 

Oyster,  Muscle,  Ascidia. 

Lingula,  Terebratula. 

Barnacle. 

III.  ARTICULATA. 

1.  Annelida, 

Serpula,  Sabella,  Amphitrite. 
Nereis,  Aphrodite,  Loh-worm. 
Earth-worm,  Leech,  Nais,  Hair-worm.. 

2.  Crustacea. 

Crab,  Lobster,  Prawn. 

Squill,  Phyllosoma. 

Gammarus,  Sand-hopper. 

Cyamus. 

Wood-louse. 

Monoculus. 

3.  Arachnid  A. 

Spider,  Tarantula,  Scorpion.. 
Phalangium,  Mite. 

4.  Insecta. 

Centipede,  Podura. 
Beetle,  Glow-worm.. 
Grasshopper,  Locust. 
Firefly,  Aphis. 
Dragon-fly,  Ephemera. 
Bee,  Wasp,  Jint. 
Butterfly,  Moth. 
Xenos,  Sty  lops. 
Gnat,  House-fly. 

IV.  ZOOPHYTA. 

Star-fish,  Urchin. 

Fluke,  Hydatid,  Tape-worm. 

Actinia,  Medusa. 

Hydra,  Coral,  Madrepore,  Pennatula. 

Brachionus,  Vibrio,  Proteus,  Monas. 


«•  C;  State  College 


ANIMAL   AND   VEGETABLE 


PHYSIOLOGY. 


INTRODIJCTIOIV. 

CHAPTER  I. 

FINAL    CAUSES. 

To  investigate  the  relations  which  connect  Man  with  his 
Creator  is  the  noblest  exercise  of  human  reason.  The  Be- 
ing who  bestowed  on  him  this  faculty  cannot  but  have  in- 
tended that  he  should  so  exercise  it,  and  that  he  should  ac- 
quire, through  its  means,  some  insight,  however  limited, 
into  the  order  and  arrangements  of  creation;  some  know- 
ledge, however  imperfect,  of  the  divine  attributes;  and  a 
distinct,  though  faint,  perception  of  the  transcendent  glory 
with  which  those  attributes  are  encompassed.  To  Man 
have  been  revealed  the  power,  the  w^isdom,  and  the  good- 
ness of  God,  through  the  medium  of  the  Book  of  Nature, 
in  the  varied  pages  of  which  they  are  inscribed  in  indelible 
characters.  On  Man  has  been  conferred  the  high  privilege 
of  interpreting  these  characters,  and  of  deriving  from  their 
contemplation  those  ideas  of  grandeur  and  sublimity,  and 
those  emotions  of  admiration  and  of  gratitude,  which  ele- 
vate and  refine  the  soul,  and  transport  it  into  regions  of  a 
purer  and  more  exalted  being. 

Vol.  I.  3 


18  FINAL  CAUSES. 

A  study  which  embraces  so  extensive  a  range  of  objects, 
and  which  involves  questions  of  such  momentoue  interest 
to  mankind,  must  necessarily  be  arduous,  and  requires  for 
its  successful  prosecution  the  strenuous  exertions  of  the  hu- 
man intellect,  and  the  combined  labours  of  different  classes 
of  philosophers,  during  many  ages.  The  magnitude  of  the 
task  is  increased  by  the  very  success  of  those  previous  ef- 
forts: for  the  difficulties  augment  as  the  objects  multiply, 
and  the  eminence  on  which  the  accumulated  knowledge  of 
centuries  has  placed  us  only  discloses  a  wider  horizon,  and 
the  prospect  of  more  fertile  regions  of  inquiry;  till  at  length 
the  mind,  conscious  of  the  inadequacy  of  its  own  powers  to 
the  comprehension  of  even  a  small  part  of  the  system  of  the 
universe,  is  appalled  by  the  overwhelming  consideration  of 
the  infinity  that  surrounds  us.  The  reflection  continually 
presents  itself  that  the  portion  of  creation  we  are  here  per- 
mitted to  behold  is  as  nothing,  when  compared  with  the 
immensity  of  space,  which,  on  every  side,  spreads  far  be- 
yond the  sphere  of  our  vision,  and,  indeed,  far  beyond  the 
powers  of  human  imagination.  Of  the  planetary  system, 
which  includes  this  earth,  our  knowledge  is  almost  entirely 
limited  to  the  mathematical  laws  that  regulate  the  motions 
of  the  bodies  which  compose  it,  and  to  the  celestial  me- 
chanism which  patient  investigation  has  at  length  discovered 
to  be  that  most  admirably  calculated  to  preserve  their  har- 
mony and  maintain  their  stability.  Still  less  have  we  the 
means  of  penetrating  into  the  remoter  regions  of  the  heavens, 
where  the  result  of  our  investigations  respecting  the  myriads 
of  luminous  bodies  they  contain  amounts  to  little  more  than 
the  knowledge  of  their  existence,  of  their  countless  num- 
bers, and  of  the  immeasurable  distances  at  which  they  are 
dispersed  throughout  the  boundless  realms  of  space. 

Measured  on  the  vast  scale  of  the  universe,  the  globe  we 
inhabit  appears  but  as  an  atom;  and  yet,  within  the  compass 
of  this  atom,  what  an  inexhaustible  variety  of  objects  is  con- 
tained: what  an  endless  diversity  of  phenomena  is  presented; 
what  wonderful  changes  are  occurring  in  rapid  and  perpetual 


PINAL  CAUSES.  19 

succession!     Throughout  the  whole  series  of  terrestrial  be- 
ings, what  studied  arrangements,  what  preconcerted  adapta- 
tions, what  multiplied  evidences  of  intention,  what  signal 
proofs  of  beneficent  design  exist  to  attract  our  notice,  to  ex- 
cite our  curiosity,  and  to  animate  our  inquiries.     Splendid 
as  are  the  monuments  of  divine  power  and  wisdom  displayed 
throughout  the  firmament,  in  objects  fitted  by  their  stupen- 
dous magnitude  to  impress  the  imagination  and  overpower 
us  by  their  awful  grandeur,  not  less  impressive,  nor  less  re- 
plete with  wonder,  are  the  manifestations  of  those  attributes 
in  the  minuter    portions  of  nature,  which  are  more  on  a 
level  with  our  senses,  and  more  within  the  reach  of  our 
comprehension.      The   modern    improvements    of   optical 
science,  which  have  expanded  our  prospects  into  the  more 
distant  regions  of  the  universe,  have  likewise  brought  with- 
in our  range  of  vision  the  more  diminutive  objects  of  crea- 
tion, and  have  revealed  to  us  many  of  the  secrets  of  their 
structure  and  arrangement.     But,  farther,  our  reason  tells 
us  that,  from  the  infinite  divisibility  of  space,  there  still  exist 
worlds  far  removed  from  the  cognizance  of  every  human 
sense,  however  assisted  by  the  utmost  refinements  of  art; 
worlds  occupied   by  the  elementary  corpuscles  of  matter, 
composing,  by  their  various  configurations,  systems  upon 
systems,  and  comprising  endless  diversities  of  motions,  of 
complicated    changes,   and    of  widely   extended    series  of 
causes  and  effects,  destined  for  ever  to  remain  invisible  to 
human  eyes,  and  inscrutable  to  human  science. 

Thus,  in  whatever  field  we  pursue  our  inquiries,  we  are 
sure  to  arrive  at  boundaries  within  which  our  powers  are 
circumscribed.  Infinity  meets  us  in  every  direction,  whe- 
ther in  the  ascending  or  descending  scale  of  magnitude;  and 
we  feel  the  impotence  of  our  utmost  efforts  to  fathom  the 
depths  of  creation,  or  to  form  any  adequate  conception  of 
that  supreme  and  Dominant  Intelligence,  which  compre- 
hends the  whole  chain  of  being  extending  from  that  which 
is  infinitely  small  to  that  which  is  infinitely  great. 

It  is  incumbent  on  us,  before  ene;aging  in  a  study  of  such 


20  PINAL  CAUSES. 

vast  importance,  and  extending  over  so  wide  a  field  as  that 
which  lies  before  us,  to  examine  with  attention  the  nature 
of  those  processes  of  reasoning,  by  which  we  are  conducted 
to  the  knowledge  of  the  peculiar  class  of  truths  we  are  seek- 
ing. Such  a  preliminary  inquiry  is  the  more  necessary,  in- 
asmuch as  the  investigation  of  these  truths  is  beset  wdth 
many  formidable  difficulties  and  liable  to  various  sources  of 
fallacy,  which  are  not  met  with  in  the  study  of  other  depart- 
ments of  philosophy. 

The  proper  objects  of  all  human  knowledge  are  the  rela- 
tions that  exist  among  the  phenomena  of  which  the  mind 
has  cognizance.  The  phenomena  of  the  universe  may  be 
viewed  as  connected  with  one  another  either  by  the  relation 
of  cause  and  effect,  or  by  that  of  means  and  end;  and,  ac- 
cordingly, these  two  classes  of  relations  give  rise  to  different 
kinds  of  knowledge,  each  of  which  requires  to  be  inves- 
tigated in  a  peculiar  mode  and  by  a  different  process  of  rea- 
soning. The  foundation  of  both  these  kinds  of  knowledge 
is,  indeed,  the  same;  namely,  the  constant  uniformity  which 
takes  place  in  the  succession  of  events,  and  which,  when 
traced  in  particular  classes  of  phenomena,  constitutes  what 
we  metaphorically  call  the  Laws  of  Nature.  It  is  the  pro- 
vince of  philosophy,  strictly  so  called,  to  discover  the  cir- 
cumstances or  laws  which  regulate  this  uniformity,  and  to 
arrange  the  observed  changes  according  to  their  invariable 
antecedents,  or  causes:  the  unknown  links  by  which  these 
causes  are  connected  with  their  respective  consequents,  or 
effects,  being  denominated  the  powers  of  Nature.  With 
reference  to  phenom.ena  which  are  purely  mechanical,  that 
is,  to  changes  which  consist  in  the  sensible  motions  of  ma- 
terial  bodies,  these  powers  are  denominated /orce.s;  and  the 
intensities,  the  operations,  and  the  characters  of  these  forces 
admit  of  exact  definition,  according  to  the  qualities  of  the 
corresponding  efiects  they  produce.  It  is  by  pursuing  the 
method  of  philosophical  induction,  so  well  explained  by  Ba- 
con, that  the  physical  sciences,  which  the  misdirected  efTorts 
of  former  ages  had  failed  to  advance,  have,  within  the  last 


FINAL  CAUSES.  21 

two  centuries,  been  carried  to  a  height  of  perfection  which 
affords  just  grounds  for  exultation  in  the  achievements  of  the 
human  intellect. 

In  the  investigation  of  the  powers  which  are  concerned  in 
the  phenomena  of  living  beings,  we  meet  with  diflficulties  in- 
comparably greater  than  those  that  attend  the  discovery  of 
the  physical  forces  by  which  the  parts  of  inanimate  matter 
are  actuated.  The  elements  of  the  inorganic  world  are  few 
and  simple;  the  combinations  they  present  are  in  most  cases 
easily  unravelled;  and  the  powers  which  actuate  their  mo- 
tions, or  effect  their  union  and  their  changes,  are  reducible 
to  a  small  number  of  general  laws,  of  which  the  results  may, 
for  the  most  part,  be  anticipated,  and  exactly  determined  by 
calculation.  What  law,  for  instance,  can  be  more  simple 
than  that  of  gravitation,  to  which  all  material  bodies,  what- 
ever be  their  size,  figure,  or  other  properties,  and  whatever 
be  their  relative  positions,  are  equally  subjected;  and  of 
which  the  observations  of  modern  astronomers  have  rendered 
it  probable  that  the  influence  extends  to  the  remotest  regions 
of  space?  The  most  undeviating  regularity  is  exhibited  in 
the  motions  of  those  stupendous  planetary  masses,  which 
continually  roll  onwards  in  the  orbits  prescribed  by  this  all- 
pervading  force.  Even  the  slighter  perturbations  occasioned 
by  their  mutual  influence  are  but  direct  results  of  the  same 
general  law,  and  are  necessarily  restrained  within  certain 
limits,  which  they  never  can  exceed,  and  by  which  the  per- 
manence of  the  system  is  effectually  secured.  All  the  ter- 
restrial changes  dependent  on  these  motions  partake  of  the 
same  constancy.  The  same  periodic  order  governs  the  suc- 
cession of  day  and  night,  the  rise  and  fall  of  the  tides,  and 
the  return  of  the  seasons:  which  order,  as  far  as  we  can  per- 
ceive, is  incapable  of  being  disturbed  by  any  existing  cause. 

Equally  definite  are  the  operations  of  the  forces  of  cohe- 
sion, of  elasticity,  or  of  whatever  other  mechanical  powers 
of  attraction  or  repulsion  there  may  be,  which  actuate,  at  in- 
sensible distances,  the  particles  of  matter.  We  see  liquids, 
in  obedience  to  these  forces,  collecting  in  spheroidal  masses, 


22  FINAL  CAUSES. 

or  assuming,  at  their  contact  with  solids,  certain  curvilinear 
forms,  which  are  susceptible  of  precise  mathematical  deter- 
mination. In  different  circumstances,  again,  we  behold 
these  particles  suddenly  changing  their  places,  marshalling 
themselves  in  symmetric  order,  and  constructing  by  their 
union  solid  crystals  of  determinate  figure,  having  all  their 
angles  and  facets  shaped  with  mathematical  exactness. 

The  forces  by  which  dissimilar  particles  are  united  into 
a  chemical  compound  have  been  termed  Chemical *Bffinities; 
and  the  operation  of  these  peculiar  forces  is  as  definite  and 
determinable  as  the  former.  They  are  now  known  to  be 
regulated  by  the  law  of  definite  proportions;  a  law,  the  dis- 
covery of  which  has  conferred  on  Chemistry  the  same  cha- 
racter of  precision  which  appertains  to  the  exact  sciences, 
and  which  it  had  never  before  attained.  The  phenomena  of 
Light,  of  Heat,  of  Electricity,  and  of  Magnetism  have  been, 
in  like  manner,  reduced  to  laws  of  sufficient  simplicity  to 
admit  of  the  application  of  mathematical  reasoning,  and  to 
furnish  the  accurate  results  derived  from  such  application. 

Thus,  to  whatever  department  of  physical  science  our  re- 
searches have  extended,  we  every  where  meet  with  the 
same  regularity  in  the  phenomena,  the  same  simplicity  in 
the  laws,  and  the  same  uniformity  in  the  results.  All  is 
strictly  defined,  and  subjected  to  rigid  rule:  all  is  subordi- 
nate to  one  pervading  principle  of  order.  The  great  Creator 
of  the  universe  has  exercised  in  its  construction  the  severest 
and  most  refined  geometry,  has  traced  with  unerring  precision 
the  boundaries  of  all  its  parts,  and  has  prescribed  to  each 
element  and  each  power  its  respective  sphere  and  limit. 

Far  different  is  the  aspect  of  living  Nature.  The  specta- 
cle here  offered  to  our  view  is  every  where  characterized  by 
boundless  variety,  by  inscrutable  complexity,  by  perpetual 
mutation.  Our  attention  is  solicited  to  a  vast  multiplicity 
of  objects,  curious  and  intricate  in  their  mechanism,  exhibit- 
ing peculiar  movements,  actuated  by  new  and  unknown 
powers,  and  gifted  with  high  and  refined  endowments.  In 
place  of  the  simple  combinations  of  elements,  and  the  sim- 


FINAL  CAUSES.  23 

pie  properties  of  mineral  bodies,  all  organic  structures,  even 
the  most  minute,  present  exceedingly  complicated  arrange- 
ments, and  a  prolonged  succession  of  phenomena,  so  varied 
and  so  anomalous,  as  to  be  utterly  irreducible  to  the  known 
laws  which  govern  inanimate  matter.  Let  us  hasten,  with 
fresh  ardour,  to  explore  this  new  world  that  here  opens  to 
our  view. 

Turning,  then,  from  the  examination  of  the  passive  ob- 
jects of  the  material  world,  we  now  direct  our  attention  to 
the  busy  theatre  of  animated  existence,  where  scenes  of  won- 
der and  enchantment  are  displayed  in  endless  variety  around 
us;  where  life  in  its  ever-changing  forms  meets  the  eye  in 
every  region  to  which  our  researches  can  extend;  and  where 
every  element  and  every  clime  is  peopled  by  multitudinous 
races  of  sensitive  beings,  who  have  received  from  the  boun- 
teous hand  of  their  Creator  the  gift  of  existence  and  the  means 
of  enjoyment.  Our  curiosity  is  powerfully  excited  by  pheno- 
mena in  which  our  own  welfare  is  so  intimately  concerned, 
as  are  all  those  that  relate  to  animal  life;  and  we  cannot  but 
take  a  lively  and  sympathetic  interest  in  the  history  of  be- 
ings in  many  respects  so  analogous  to  ourselves,  like  us  pos- 
sessing powers  of  spontaneous  action,  impelled  by  passions 
and  desires,  and  endowed  with  capacities  of  enjoyment  and 
of  suffering.  Can  there  be  a  more  gratifying  spectacle  than 
to  see  an  animal  in  the  full  vigour  of  health,  and  the  free 
exercise  of  its  powers,  disporting  in  its  native  element,  re- 
velling in  the  bliss  of  existence,  and  testifying  by  its  inces- 
sant gambols  the  exuberance  of  its  joy  ? 

We  cannot  take  even  a  cursory  survey  of  the  host  of  living 
beings  profusely  spread  over  every  portion  of  the  globe, 
without  a  feeling  of  profound  astonishment  at  the  inconceiva- 
ble variety  of  forms  and  constructions  to  which  animation 
has  been  imparted  by  creative  power.  What  can  be  more 
calculated  to  excite  our  wonder  than  the  diversity  exhibited 
among  insects,  all  of  which,  amidst  endless  modifications  of 
shape,  still  preserve  their  conformity  to  one  general  plan  of 
construction  ?     The  number  of  distinct  species  of  insects 


24  FINAL  CAUSES. 

already  known  and  described  cannot  be  estimated  at  less 
tban  100,000;  and  every  day  is  adding  to  the  catalogue.* 
Of  the  comparatively  large  animals  which  live  on  land,  how 
splendid  is  the  field  of  observation  that  lies  open  to  the  na- 
turalist! What  variety  is  conspicuous  in  the  tribes  of  Quad- 
rupeds and  of  Reptiles;  and  what  endless  diversity  exists  in 
their  habits,  pursuits,  and  characters!  How  extensive  is  the 
study  of  Birds  alone;  and  how  ingeniously,  if  we  may  so 
express  it,  has  nature  interwoven  in  their  construction  every 
possible  variation  compatible  with  an  adherence  to  the  same 
general  model  of  design,  and  the  same  ultimate  reference  to 
the  capacity  for  motion  through  the  light  element  of  air. 
What  profusion  of  being  is  displayed  in  the  wide  expanse  of 
the  ocean,  through  which  are  scattered  such  various  and  such 
unknown  multitudes  of  animals !  Of  Fishes  alone  the  varie- 
ties, as  to  conformation  and  endowments,  are  endless.  Still 
more  curious  and  anomalous,  both  in  their  external  form, 
and  their  internal  economy,  are  the  numerous  orders  of 
living  beings  that  occupy  the  lowxr  divisions  of  the  animal 
scale;  some  swimming  in  countless  miyriads  near  the  sur- 
face; some  dwelling  in  the  inaccessible  depths  of  the  ocean: 
some  attached  to  shells,  or  other  solid  structures,  the  pro- 
ductions of  their  own  bodies,  and  which,  in  process  of  time, 
form,  by  their  accumulation,  enormous  submarine  moun- 
tains, rising  often  from  unfathomable  depths  to  the  surface. 
What  sublime  views  of  the  magnificence  of  creation  have 
been  disclosed  by  the  microscope  in  the  world  of  infinite 
minuteness,  peopled  by  countless  multitudes  of  atomic  be- 
ings which  animate  almost  every  fluid  in  nature  ?  Of  these, 
a  vast  variety  of  species  has  been  discovered,  each  animal- 
cule being  provided  w^ith  appropriate  organs,  endowed  with 
spontaneous  powers  of  motion,  and  giving  unequivocal  signs 
of  individual  vitality.     The  recent  observations  of  Profes- 

*  Four-fifths  of  the  insects  at  present  known  have  been  discovered  within 
the  last  ninety  years:  for  in  1743,  Ray  estimated  the  total  number  of  species 
at  20,000  only.  See  his  work  on  "  The  Wisdom  of  God  as  manifested  in  the 
Creation,"  p.  24. 


FINAL  CAUSES.  25 

sor  Ehrenberg  have  brought  to  light  the  existence  of  71//?- 
nadsy  which  are  not  larger  than  the  24,000th  of  an  inch, 
and  which  are  so  thickly  crowded  in  the  fluid  as  to  leave 
intervals  not  greater  tlian  their  own  diameter.  Hence,  he 
has  made  the  computation  that  each  cubic  line,  which  is 
nearly  the  bulk  of  a  single  drop,  contains  500,000,000  of 
these  monads,  a  number  which  equals  that  of  all  the  human 
beings  existing  on  the  surface  of  the  globe. 

Thus,  if  we  review  every  region  of  the  globe,  from  the 
scorching  sands  of  the  equator  to  the  icy  realms  of  the  poles, 
or  from  the  lofty  mountain  summits  to  the  dark  abysses  of 
the  deep;  if  we  penetrate  into  the  shades  of  the  forest,  or 
into  the  caverns  a^  secret  recesses  of  the  earth;  nay,  if  wc 
take  up  the  minutest  portion  of  stagnant  water,  we  still 
meet  with  life  in  some  new  and  unexpected  form,  yet 
ever  adapted  to  the  circumstances  of  its  situation.  Where- 
ver life  can  be  sustained,  we  find  life  produced.  It  would 
almost  seem  as  if  Nature^  had  been  thus  lavish  and  sportive 
in  her  productions  with  the  intent  to  demonstrate  to  Man 
the  fertility  of  her  resources,  and  the  inexhaustible  fund  from 
which  she  has  so  prodigally  drawn  forth  the  means  requisite 
for  the  maintenance  of  all  these  diversified  combinations, 
for  their  repetition  in  endless  perpetuity,  and  for  their  su- 
bordination to  one  harmonious  scheme  of  general  grood. 

The  vegetable  world  is  no  less  prolific  in  wonders  than 
the  animal.  In  this,  as  in  all  other  parts  of  creation,  ample 
scope  is  found  for  the  exercise  of  the  reasoning  faculties; 
and  at  the  same  time  abundant  sources  are  supplied  of  intel- 
lectual enjoyment.  To  discriminate  the  different  characters 
of  plants,  amidst  the  infinite  diversity  of  shape,  of  colour,  and 

•  In  order  to  avoid  thee  too  frequent,  and  consequently  irreverent,  intro- 
duction of  ihe  Great  Name  of  the  Supkkxk  Ukixr  into  familiar  discourse  on 
the  operations  of  his  power,  I  have,  throughout  this  Treatise,  followed  the 
common  usag-e  of  employing-  the  term  Nature  as  a  synonym,  expressive  of 
the  same  power,  but  veiling-  fromoiu*  feeble  sight  the  too  dazzling-  spleudom* 
of  its  glory. 

Vol.  I.  4 


26  FINAL  CAUSES. 

of  structure,  which  they  offer  to  our  observation,  is  the  la- 
borious, yet  fascinating,  occupation  of  the  Botanist.     Here, 
also,  we  are  lost  in  admiration  at  the  never-ending  variety 
of  forms  successively  displayed  to  view  in  the  innumerable 
species  which   compose   this  kingdom  of  nature,  and  at  the 
energy  of  that  vegetative  power,  which,  amidst  such  great 
differences  of  situation,  sustains  the  modified  life  of  each  in- 
dividual plant,  and  which  continues  its  species  in  endless 
perpetuity.     Wherever  circum.stances  are  compatible  with 
vegetable  existence,  we  there  find  plants  arise.     It  is  well 
known  that,  in    all  places  where  vegetation  has  been  estab- 
lished, the   germs  are  so*  intermingled  with  the  soil,  that 
whenever  the  earth    is  turned  up,  even  from  considerable 
depths,  and  exposed  to  the  air,  plants   are  soon  observed  to 
spring,  as  if  they  had  been  recently  sown,  in  consequence  of 
the  o-ermi nation  of  seeds  which  had  remained  latent  and  in- 
active  during  the  lapse  of  perhaps  many  centuries.     Islands 
formed  by  coral  reefs,  which  have  risen  above  the  level  of  the 
sea,  become  in  a  short  time  covered  with  verdure.  From  the 
materials  of  the  most  steril  rock,  and  even  from  the  yet 
recent  cinders  and  lava  of  the  volcano.  Nature  prepares  the 
way  for  vegetable  existence.     The  slightest  crevice  or  ine- 
quality is  sufficient  to  arrest  the  invisible  germs  that  are  al- 
ways floating  in  the  air,  and  affords  the  means  of  sustenance 
to    diminutive  races  of  lichens  and   mosses.     These   soon 
overspread  the  surface,  and  are  followed,  in  the  course  of  a 
few   years,  by  successive   tribes   of  plants  of  gradually  in- 
creasing size  and  strength;  till  at  length  the  island,  or  other 
favoured    spot,    is    converted  into  a  natural  and  luxuriant 
garden,   of  which  the  productions    rising    from  grasses  to 
shrubs  and  trees,  present  all  the  varieties  of  the  fertile  mea- 
dow, the  tangled  thicket,  and  the  widely  spreading  forest. 
Even  in  the  desert  plains  of  the  torrid  zone,  the  eye  of  the 
traveller  is  often  refreshed  by  the  appearance  of  a  few  hardy- 
plants,  which  find   sufficient  materials  for  their  growth   in 
these  arid  regions:  and    in  the  realms   of  perpetual  snow 
which  surround  the   poles,  the   navigator   is   occasionally 


FINAL  CAUSES.  27 

startled  at  the  prospect  of  fields  of  a  scarlet  hue,  the  result 
of  a  wide  expanse  of  microscopic  vegetation.* 

But  whatever  charms  the  naturalist  may  find  in  the  oc- 
cupations in  which  he  is  engaged,  and  however  wide   may 
be  the  field  of  his  exertions,  they  still   are   insufficient  to 
satisfy  the  more  enlarged  curiosity  of  a  philosophic  mind. 
The  passive  emotion  of  astonishment,  in  which  inferior  in- 
tellects are  content   to  rest,  serves  but   to  awaken,  in  him 
who  has  learned  to   think,  a  desire   of  farther  knowledge. 
Filled  with  an  ardent  spirit  of  inquiry,  he  cannot  but  be  im- 
patient under  the  feeling  that,  while  Nature  has  placed  be- 
fore his  eyes  this  splendid  spectacle  of  animation,  she   has 
thrown  a  dense  veil  over  the  interior  machinery  of  life,  and 
has  concealed  from  his  view  the  springs  by  which  she  sets 
it  in  motion.     With  the  hope  of  discovering  her  proceed- 
ings, he  hastens  to  explore  the  several  parts  which  compose 
the  organized  fabric,  to  examine  in  minute  detail  the  anato- 
my of  its  structure,  and  to  ascertain  the  nature  of  the  seve- 
ral actions  that  take  place  within  it.     But  overwhelmed  by 
the  multiplicity    of  objects,    and   lost  amidst    the  compli- 
cation of  phenomena,  he   soon  becomes  dismayed  by  the 
magnitude    and   arduous  nature  of  the  investigation.     He 
finds  that  his  labours  w^ill  be  of  no  avail,  unless,  previous- 
ly to  any  attempt  at  theory,  he  takes  a  careful  and  accu- 
rate account  of  all  the  circumstances  attending  the  history 
and  conditions  of  life,  from  the  dawn  of  its  existence  to  its 
appointed  close.    On  tracing  living  beings  to  their  origin,  he 
learns  that  every  individual  vegetable  and  animal  takes  its 
rise  from  an  atom  of  imperceptible  minuteness,  and  gradual- 
ly increases  in  bulk  by  successive  accretions  of  new  matter, 
derived  from  foreign  sources,  and  by  some  refined,  but  un- 
known process,  transmuted  into  its  own  substance.     Then, 

*  The  red  snow,  discovered  in  Baffin's  Bay  on  the  17th  of  August,  1818, 
during-  the  Northern  Expedition,  under  the  command  of  Captain  Ross,  was 
found  to  owe  its  colour  to  minute  fung-i,  or  microscopic  mushrooms,  which 
veg-etate  on  the  surface  of  snow,  as  their  natural  abode.  See  Phil.  Trans,  for 
1820,  p.  165. 


28  FINAL  CAUSES. 

following  the  progressive  development  of  the  organs,  he  ob- 
serves them  undergoing  various  modifications,  as  they  are 
assuming  new  forms,  which  characterize  certain  definite 
epochs  in  the  general  growth  of  the  system.  In  a  great 
number  of  instances,  especially  among  the  lower  orders  of 
animals,  he  witnesses  the  same  individual  being  acting,  in 
its  time,  a  variety  of  different  parts;  often  reappearing  on 
the  stage  of  life  with  new  organs,  new  faculties,  and  new 
conditions  of  existence,  and  undergoing  metamorphoses  as 
complete  as  any  that  have  been  depicted  in  the  fables  of  an- 
tiquity. 

The  period  at  length  arrives  when  the  animal,  having 
completed  its  growth,  attains  the  maturity  of  its  being,  and 
acquires  the  full  possession  of  its  powers.  Every  organ  in 
succession  has  received  its  entire  development,  and  has 
united  its  energies  with  those  which  had  been  before  per- 
fected. Yet,  however  complete  the  arrangements  that  have 
thus  been  established,  it  is  still  necessary,  in  order  to  pre- 
serve the  whole  system  in  a  state  in  which  it  may  be  capa- 
ble of  exercising  the  functions  of  life,  th^t  the  materials 
which  compose  its  fabric  should  undergo  a  certain,  slow,  but 
constant  renovation;  and  the  same  circle  of  actions  and  re- 
actions, which  have  brought  it  to  its  state  of  perfection,  must 
continue  to  be  repeated,  in  order  that  a  due  proportion  may 
be  maintained  between  the  consumption  and  the  supply  of 
these  materials.  In  the  course  of  a  certain  time,  however, 
even  under  the  most  favourable  circumstances,  this  equili- 
brium begins  to  fail:  the  energies  of  the  system  decline: 
and  the  processes  of  nutrition  are  insufficient  to  repair  the 
waste  in  the  substance  of  the  body.  The  fluids  are  dissi- 
pated faster  than  they  can  be  renewed;  the  channels  through 
which  they  circulate  are  more  and  more  obstructed,  and  at 
length  cease  to  be  pervious:  and  the  solids  gradually  become 
hard  and  rigid.  As  in  a  machine  of  which  the  wheels  are 
worn,  and  the  springs  have  lost  their  elastic  force,  so  in  the 
animal  bod}',  at  an  advanced  age,  the  slightest  additional 
impediment  that  occurs  will  stop   the  movements  of  the 


FINAL  CAUSES.  29 

whole  system:  and.  when  once  stopped,  their  renewal  is  im- 
possible. Nature  has  thus  assigned  to  every  living  being  a 
certain  period  as  the  utmost  extent  of  its  duration.  Even 
when  exempt  from  external  interference,  all  are  doomed  to 
perish,  sooner  or  later,  by  the  slow  but  unerring  operation 
of  the  same  internal  causes  which  originally  effected  their 
development  and  growth,  and  which  are  inseparably  inter- 
woven with  the  conditions  of  their  existence. 

Numerous,  however,  are  the  extraneous  and  accidental 
causes  that  may  hasten  or  precipitate  their  destruction,  long 
before  the  period  of  natural  decay.     How  striking  is  the 
contrast,  on  those  occasions,  between  the  scene  we  have  just 
beheld  of  an  animal  in  the  full  vigour  of  its  powers,  either 
rapidly  bounding  across  the  plain,  or  gliding  beneath  the 
wave,  or  soaring  in  the  elevated  regions  of  air,  and  the  spec- 
tacle 0^  the  same  animal  lying,  the  next  moment,  extended 
at  our  feet,  bereft  at  once  of  activity  and  of  sense — of  all  the 
faculties  and  powers  that  constitute  life.     Can  we  contem- 
plate without  amazement  so  complete  and  instantaneous  a 
change;  so  sudden  and  awful  a  catastrophe?     Must  we  not 
be  animated  by  an  eager  desire  to  penetrate  so  great  a  mys- 
tery, and  resolve  the   many  questions  which  so  striking  a 
phenomenon  must  naturally  suggest?     What,  we  are  led  to 
ask,  is  the  nature  of  this  extraordinary  revolution,  extending 
over  the  whole  of  that  frame  which  had   so  long  delighted 
the  eye  by  its  beauty,  and  producing  this  sudden  and  irre- 
trievable extinction  of  the  powers  of  life?     How  comes  it 
that  all  those  mighty  energies  which    the    animal  had  so 
lately  displayed,  and  which  had  called  forth  our  admiration, 
perhaps  even  excited  our  envy,  are  at  once  and  for  ever 
annihilated?     What  was  the  bond,  thus  suddenly  dissevered, 
which  held  together  the  various  parts  of  that  compound 
frame?     What  potent  spell  has  been  dissolved,  which  could 
retain  in  combination   for  so  long  a  period  the  multifarious 
elements  of  that  exquisite  organization;  and  from  the  con- 
trol of  which  being  now  released,  these  elements  hasten  to 
resume   their   wonted   attractions,  and    entering  into   new 


30  FINAL  CAUSES. 

forms  of  combination,  are  scattered  into  dust,  or  dissipated 
in  air,  leaving  no  trace  of  their  former  union?  What  me- 
chanism  has  been  employed  in  its  construction?  What  re- 
fined chemistry  has  been  exerted  in  assimilating  new  parti- 
cles of  matter  to  those  previously  organized,  and  in  appro- 
priating them  to  the  nourishment  of  the  parts  with  which 
they  become  identified?  By  what  transcendent  power, 
above  all,  did  this  assemblage  of  material  particles  first  be- 
come animated  by  the  breath  of  life;  and  from  what  elevated 
source  did  they  derive  those  higher  energies,  apparently  so 
foreign  to  their  inherent  properties,  and  investing  these  once 
lifeless  and  inert  materials  with  the  exalted  attributes  of  ac- 
tivity, of  sensation,  of  perception,  of  intelligence?  Shall 
we  ever  comprehend  the  nature  of  this  subtle  and  pervading 
principle,  by  the  agency  of  which  all  these  wonderful  phe- 
nomena of  life  are  produced,  and  which,  combining  into  one 
harmonious  system  so  many  heterogeneous  and  jarring  ele- 
ments, has  led  to  the  formation  of  this  exquisite  frame,  this 
elaborate  machine,  this  miraculous  assemblage  of  faculties? 

The  discovery  of  a  clew,  if  any  such  can  be  found,  to  the 
mazes  of  this  perplexing  labyrinth  can  be  hoped  for  only  from 
the  successful  cultivation  of  the  science  of  physiology.  But 
before  engaging  in  this  arduous  study,  we  ought  previously 
to  inquire  into  the  methods  of  reasoning  by  which  it  is  to  be 
conducted. 

The  object  of  physiology  is,  by  the  diligent  examination 
of  the  phenomena  of  life,  to  ascertain  the  laws  which  re- 
gulate those  phenomena,  both  as  they  apply  to  the  indivi- 
dual beings  endowed  with  life,  and  also  as  they  relate  to  the 
various  assemblages  that  constitute  the  species,  the  genera, 
the  families,  the  orders,  and  the  classes  of  those  beings:  and, 
lastly,  as  they  concern  the  whole  collective  union  of  the 
organized  world. 

These  peculiar  laws,  which  it  is  the  province  of  physio- 
logy to  investigate,  are,  as  I  have  before  observed,  of  two 
kinds,  each  founded  upon  relations  of  a  different  class.  The 
first,  which  depend  upon  the  simple  relation  of  cause  an'd  ef- 


FINAL  CAUSES.  31 

feet,  are  concerned  merely  with  the  natural  powers  of  mat- 
ter. They  are  the  laws  that  regulate  the  succession  of  pheno- 
mena purely  physical  in  all  their  stages.  These  phenome- 
na consist  in  changes  among  material  particles,  which  are 
either  of  a  meclianical  or  chemical  nature;  or  in  the  affec- 
tions of  imponderable  pliysical  agents,  such  as  heat,  light-, 
electricity,  and  magnetism;  and  they  include  also  the  pheno- 
mena that  take  place  in  organized  bodies,  and  which  are  re- 
ferrible  to  the  operation  of  certain  physical  powers,  apper- 
taining to  particular  structures,  such  as  muscular  contraction 
and  nervous  irritation;  phenomena  which,  as  we  shall  after- 
wards find,  are  not  reducible  to  any  of  the  former  laws,  but 
are  peculiar  to  the  living  state.  The  second  class  of  laws 
comprise  those  which  are  founded  on  the  relation  of  means 
to  an  end;  and  which  are  usually  denominated  Jifia  I  causes. 
They  involve  the  operations  of  mind,  in  conjunction  with 
those  of  matter.  They  presuppose  intention  or  design;  a 
supposition  which  implies  intelligence,  thought,  motives,  vo- 
lition,— particular  purposes  to  be  answered,  requiring  the 
agency  of  powers  and  of  instruments  adapted  to  the  produc- 
tion of  the  intended  efiects: — the  knowledge  of  the  proper- 
ties of  matter,  the  selection  and  choice  of  particular  means, 
and  the  power  of  employing  them  in  an  effective  manner. 
These  purposes  may  themselves  be  subservient  to  more  ge- 
neral objects,  and  these  objects  again  subordinate  to  remoter 
ends:  so  that  the  whole  shall  comprehend  a  systematic  plan 
of  operations,  conducive,  on  the  most  enlarged  views,  to  ul- 
timate and  general  utility. 

The  study  of  these  final  causes  is.  In  some  measure,  forced 
upon  our  attention  by  even  the  most  superficial  survey  of 
nature.  It  is  impossible  not  to  recognise  the  character  of 
intention,  which  is  so  indelibly  impressed  upon  every  part 
of  the  structure  both  of  vegetable  and  animal  beings,  and 
which  marks  the  whole  series  of  phenomena  connected 
with  their  history.  Microscopic  observations  teach  us 
that  the  embryo  of  an  organic  being  contains  within  it- 
self the  rudiments  of  the  future  vegetable  or  animal  struc- 


32  FINAL  CAUSES. 

ture,  into  which  it  is  gradually  transformed  by  the  slow 
and  successive  expansion  and.  development  of  all  its  parts. 
The  processes  of  nutrition  do  nothing  more  than  fill  up 
the  outlines  already  sketched  on  the  living  canvas.  Every 
organ,  nay  every  fibre,  resulting  from  this  development, 
contributes  its  share  in  the  production  of  certain  definite  ef- 
fects, which  we  constantly  witness  taking  place  around  us, 
as  well  as  experience  in  our  own  persons.  But  these  effects, 
though  so  familiar  to  us,  are  not  on  that  account  the  less  in- 
volved in  mystery,  or  the  less  replete  with  wonder.  To 
say  that  they  are  the  results  of  chance  conveys  no  informa- 
tion; and  is  equivalent  to  the  assertion  that  they  are  wholly 
without  a  cause.  Every  one  who  is  accustomed  to  reflect 
upon  the  operations  of  his  own  mind,  must  feel  that  such  a 
conclusion  is  contrary  to  the  constitution  of  human  thought; 
for  if  we  are  to  reason  at  all,  we  can  reason  only  upon  the 
principle  that  for  every  effect  there  must  exist  a  correspond- 
ing cause;  or,  in  other  words,  that  there  is  an  established 
and  invariable  order  of  sequence  among  the  changes  which 
take  place  in  the  universe. 

But  though  it  be  granted  that  all  the  phenomena  we  be- 
hold are  the  efiects  of  certain  causes,  it  might  still  be  alleged, 
as  a  bar  to  all  farther  reasoning,  that  these  causes  are  not 
only  utterly  unknown  to  us,  but  that  their  discovery  is 
wholly  beyond  the  reach  of  our  faculties.  The  argument  is 
specious  only  because  it  is  true  in  one  particular  sense,  and 
that  a  very  limited  one.  Those  who  urge  it,  do  not  seem 
to,  be  aware  that  its  general  application,  in  that  very  same 
sense,  would  shake  the  foundation  of  every  kind  of  know- 
ledge, even  that  which  we  regard  as  built  upon  the  most 
solid  basis.  Of  causation,  it  is  agreed  that  we  know  nothing: 
all  that  we  do  know  is,  that  one  event  succeeds  another  with 
undeviating  constancy.  Now,  if  we  were  to  probe  this  sub- 
ject  to  the  bottom,  we  should  find  that,  in  rigid  strictness, 
we  have  no  certain  knowledge  of  the  existence  of  any  thing, 
save  that  of  the  sensations  and  ideas  which  are  actually  pass- 
ing in  our  minds,  and  of  which  we  are  necessarily  con- 


FINAL  CAUSE*  33 

scious.  Our  belief  in  the  existence  of  external  objects,  in 
their  undergoing  certain  changes,  and  in  their  possessing 
certain  physical  properties,  rests  on  a  different  foundation, 
namely,  the  evidence  of  our  senses;  for  it  is  the  result  of  in- 
ferences which  the  mind  is,  by  the  constitution  of  its  frame, 
necessarily  led  to  form.  We  may  trace  to  a  similar  origin 
the  persuasion,  irresistibly  forced  upon  us,  that  there  exist 
not  only  other  material  objects  beside  our  own  bodies,  but 
also  other  intellectual  beings  beside  ourselves.  We  can 
neither  see  nor  feel  those  extraneous  intellects,  any  more 
than  we  can  see  or  feel  the  cause  of  gravitation,  or  the  sub- 
tle sources  of  electricity  or  magnetism.  We  nevertheless 
believe  in  the  reality  both  of  the  one  and  of  the  other;  but 
it  is  only  because  we  infer  their  existence  from  particular 
trains  of  impressions  made  upon  our  senses,  of  which  im- 
pressions alone  our  knowledge  can,  in  metaph3-sical  strict- 
ness, be  termed  certain. 

Upon  what  evidence  do  I  conclude  that  I  am  not  a  solitary 
being  in  the  universe;  that  all  is  not  centred  in  myself;  but 
that  there  exist  other  intellects  similar  to  my  own?  Un- 
doubtedly no  other  than  the  observation  that  certain  effects 
are  produced,  which  the  experience  I  have  had  of  the  ope- 
rations of  my  own  mind  lead  me,  by  an  irresistible  analogy, 
to  ascribe  to  a  similar  agency,  emanating  from  other  beings; 
beings,  however,  of  whose  actual  intellectual  presence  I  can- 
not be  conscious,  whose  nature  I  cannot  fathom,  whose  es- 
sence I  cannot  understand.  I  can  judge  of  the  operations  of 
other  minds  only  in  as  far  as  those  operations  accord  with 
what  has  passed  in  my  own.  I  cannot  divine  processes  of 
thought  to  which  mine  have  borne  no  resemblance,  I  can- 
not appreciate  motives  of  which  I  have  never  felt  the  in- 
fluence, nor  comprehend  the  force  of  passions,  never  yet 
awakened  in  my  breast:  neither  can  I  picture  to  myself  fecl# 
ings  to  which  no  sympathetic  chord  within  me  has  ever  vi- 
brated. 

Our  own  intelligence,  our  own  views,  and  our  own  affec- 
tions, then,  furnish  the  only  elements  by  which  it  is  possible 

Vol.  I.  5 


34  FINAL  CAUSES. 

for  us  to  estimate  the  analogous  powers  and  attributes  of 
other  minds.  The  difficulty  of  applying  this  scale  of  mea- 
surement will,  of  course,  increase  in  proportion  to  the  dif- 
ference between  the  objects  compared;  and  although  we  may 
conceive  that  there  are  powers  and  intelligences  infinitely 
surpassing  our  own,  the  conceptions  we  can  form  of  such 
superior  essences  must  necessarily  be  indefinite  and  obscure, 
and  must  partake  of  the  same  kind  of  imperfection  as  our 
notions  of  the  distances  of  the  heavenly  bodies,  however 
familiar  we  may  be  with  the  units  of  the  scale  by  which 
those  distances  are  capable  of  being  expressed.  When,  on 
the  other  hand,  the  objects  contemplated  are  more  within 
the  range  of  our  mental  vision;  when,  for  instance,  they  are 
phenomena  that  we  can  assimilate  to  our  own  voluntary 
acts,  and  in  which  we  can  clearly  trace  the  connexion  be- 
tween means  and  end,  then  does  our  recognition  of  the 
agency  of  intellect  become  most  distinct,  and  our  conviction 
of  its  real  and  independent  existence  become  most  intimate 
and  assured. 

Such  is  the  kind  of  evidence  on  which  rests  our  belief  of 
the  existence  of  our  fellow  men.  Such,  also,  is  the  founda- 
tion of  our  assurance  that  there  exists  a  mighty  Intellect, 
who  has  planned  and  executed  the  stupendous  works  of  crea- 
tion, with  a  skill  surpassing  our  utmost  conceptions;  by 
powers  to  which  we  can  assign  no  limit,  and  the  object  of 
whose  will  is  universal  good.* 

It  will  argue  no  undue  presumption,  therefore,  if,  in  our 
earnest  endeavours  to  form  just  ideas  of  the  attributes  of  the 
Deity  from  the  examination  of  nature,  we  are  led  to  insti- 
tute comparisons  between  His  works  and  those  of  man;  and 
strive  to  gather  some  faint  notions  of  the  divine  intelligence 
by  applying  the  only  standard  of  admeasurement  wdiich  we 
^ssess,  and  are  permitted  to  employ,  namely,  that  derived 
from  the  operations  of  human  intellect.  Our  interpretations 
of  the  designs  of  the  Creator  must  here  be  obtained  through 

*  The  view  here  taken  is,  of  course,  limited  to  Natural  Theology ;  that 
being'  the  express  and  exclusive  object  of  these  Treatises. 


FINAL  CAUSES.  35 

the  medium  of  human  views;  and  our  judgment  of  his  bene- 
volence can  be  formed  only  by  reference  to  our  own  affec- 
tions, and  by  their  accordance  with  those  ardent  aspirations 
after  good,  which  the  Author  of  our  being  has  deeplyinter- 
woven  with  our  frame. 

The  evidence  of  design  and  contrivance  in  the  works  of 
nature  carries  with  it  the  greatest  force  whenever  we  can 
trace  a  coincidence  between  them  and  the  products  of  hu- 
man art.  If  in  any  unknown  region  of  the  earth  we  chanced 
to  discover  a  piece  of  machinery,  of  which  the  purpose  was 
manifest,  we  should  not  fail  to  ascribe  it  to  the  workman- 
ship of  some  mechanist,  possessed  of  intelligence,  actuated 
,by  a  motive,  and  guided  by  intention.  Farther,  if  we  had 
a  previous  experience  of  the  operation  of  similar  kinds  of 
mechanism,  we  could  not  doubt  that  the  effect  we  saw  pro- 
duced was  the  one  intended  by  the  artificer.  Thus,  if  in 
an  unexplored  country,  we  saw^,  moving  upon  the  waters  of 
a  lake,  the  trunk  of  a  tree,  carved  into  the  shape  of  a  boat, 
we  should  immediately  conclude  that  this  form  had  been 
given  to  it  for  the  purpose  of  enabling  it  to  float.  If  we 
found  it  also  provided  with  paddles  at  its  sides,  we  should 
infer,  from  our  previous  knowledge  of  the  eff'ects  of  such  in- 
struments, that  they  were  intended  to  give  motion  to  this 
boat,  and  we  should  not  hesitate  to  conclude  that  the  whole 
was  the  work  of  human  hands,  and  the  product  of  human 
intelligence  and  design.  If,  in  addition,  we  found  this  boat 
furnished  with  a  rudder  and  with  sails,  we  should  at  once 
understand  the  object  of  these  contrivances,  and  our  ideas 
of  the  skill  of  the  artificer  would  rise  in  proportion  to  the 
excellence  of  the  apparatus,  and  the  ingenuity  displayed  in 
its  adaptation  to  circumstances. 

Let  us  suppose  that  in  another  part  of  this  lake  we  found 
an  insect,*  shaped  like  the  boat,  and  moving  through  the 
water  by  successive  impulses  given  to  that  medium  by  the 
action  of  levers,  extending  from  its  sides,  and  shaped  like 
paddles,  having  the  same  kind  of  movement,  and  producing 

*  Such  as  the  Notoneda  glanco,  Lin.,  or  water  boatman,  and  the  Dytiscus 
marginalis,  or  water  beetle. 


36  FINAL  CAUSES. 

the  same  efifects.  Could  we  resist  the  persuasion  that  the 
Artificer  of  this  insect,  when  forming  it  of  this  shape,  and 
providing  it  with  these  paddles,  had  the  same  mechanical 
objects  in  view?  Shall  we  not  be  confirmed  in  this  idea 
on  finding  that  these  paddles  are  constructed  with  joints, 
that  admit  no  other  motion  than  that  of  striking  against  the 
water,  and  of  thus  urging  forwards  the  animal  in  its  passage 
through  that  dense  and  resisting  medium?  Many  aquatic 
animals  are  furnished  with  tails  which  evidently  act  as  rud- 
ders, directing  the  course  of  their  progressive  motion  through 
the  fluid.  Who  can  doubt  but  that  the  same  intention  and  the 
same  mechanical  principles  which  guide  the  practice  of  the 
ship-builder,  are  here  applied  in  a  manner  still  more  refined 
and  with  a  master's  hand?  If  Nature  has  furnished  the  nau- 
tilus with  an  expansible  membrane,  which  the  animal  is 
able  to  spread  before  the  breeze,  when  propitious,  and  by 
means  of  which  it  is  wafted  along  the  surface  of  the  sea,  but 
which  it  quickly  retracts  in  unfavourable  circumstances,  is 
not  her  design  similar  to  that  of  the  human  artificer,  when 
he  equips  his  bark  with  sails,  and  provides  the  requisite 
machinery  for  their  being  hoisted  or  furled  with  ease  and 
expedition? 

The  maker  of  an  hydraulic  engine  places  valves  in  par- 
ticular parts  of  its  pipes  and  cisterns,  with  a  view  to  pre- 
vent the  retrograde  motion  of  the  fluids  which  are  to  pass 
through  them.  Can  the  valves  of  the  veins,  or  of  the  lym- 
phatics, or  of  the  heart  have  a  difierent  object:  and  are  they 
not  the  result  of  deliberate  and  express  contrivance  in  the 
great  Mechanist  of  the  living  frame? 

The  knowledge  of  the  laws  of  electricity,  in  its  difierent 
forms,  is  one  of  the  latest  results  which  science  has  revealed 
to  man.  Could  these  laws,  and  their  various  combinations, 
have  been  unknown  to  the  Power  who  created  the  torpedo, 
and  who  armed  it  with  an  energetic  galvanic  battery,  con- 
structed upon  the  most  refined  scientific  principles,  for  the 
manifest  purpose  of  enabling  the  animal  to  strike  terror  into 
its  enemies,  and  paralyze  their  efforts  to  assail  it. 

Does  not  the  optician,  who  designedly  places  his  convex 


FINAL  CAUSES.  37 

lens  at  the  proper  distance  in  a  darkened  box,  for  the  pur- 
pose of  obtaining  vivid  pictures  of  the  external  scene,  evince 
his  knowledge  of  the  laws  of  light,  of  the  properties  of  re- 
fracting media,  and  of  the  refined  combinations  of  those  me- 
dia by  which  each  pencil  is  brought  to  a  separate  focus,  and 
adjusted  to  form  an  image  of  remote  objects  ?  Does  it  not, 
in  like  manner,  argue  the  most  profound  knowledge  and 
foresight  in  the  divine  Artist,  who  has  so  admirably  hung 
the  crystalline  lens  of  the  eye  in  the  axis  of  a  spherical 
case,  in  the  forepart  of  which  He  has  made  a  circular  win- 
dow for  the  light  to  enter,  and  spread  out  on  the  opposite 
side  a  canvas  to  receive  the  picture  ?  Has  no  thought  been 
exercised  in  darkening  the  walls  of  this  camera  obscura,  and 
thus  preventing  all  reflection  of  the  scattered  rays,  that 
might  interfere  with  the  distinctness  of  the  image  ? 

But  we  farther  observe  in  the  eye  many  exquisite  refine- 
ments of  construction,  by  which  various  defects,  unavoida- 
ble in  all  optical  instruments  of  human  workmanship,  are 
remedied.  Of  this  nature  are  those  which  render  the  orjian 
achromatic,  which  correct  the  spherical  aberration,  and  which 
provide  for  the  adjustment  of  its  refracting  powers  to  the 
different  distances  of  the  objects  viewed;  not  to  speak  of  all 
the  external  apparatus  for  the  protection,  the  preservation, 
and  the  movements  of  the  e3^e-ball.  and  for  contributing  in 
every  way  to  the  proper  performance  of  its  office.  Are  not 
all  these  irrefragable  proofs  of  the  continuity  of  the  same 
design;  and  are  they  not  calculated  still  farther  to  exalt  our 
ideas  of  the  Divine  Intelligence,  of  the  elaborate  perfection 
impressed  upon  His  works;  and  of  the  comprehensive  views 
of  His  providence  ? 

These  facts,  if  they  stood  alone,  would  be  sufficient  to 
lead  us  irresistibly  to  this  conclusion:  but  evidence  of  a  si- 
milar kind  may  be  collected  in  abundance  from  every  part 
of  living  nature  to  which  our  attention  can  be  directed,  or 
to  which  our  observations  have  extended.  The  truths  they 
teach  not  only  acquire  confirmation  by  the  corroborating 
tendency  of  each  additional  fact  of  the  same  description,  but 
the  multitude  of  these  facts  is  so  great,  that  the  general  con- 


3S  FINAL  CAUSES. 

elusion  to  which  they  lead  must  be  considered  as  indubita- 
ble. For  the  argument,  as  it  has  been  justly  remarked,  is 
cumulative;  that  obtained  from  one  source  being  strength- 
ened by  that  derived  from  another;  and  all  tending  to  the 
same  conclusion,  like  rays  converging  to  the  same  point, 
on  which  they  concentrate  their  united  powers  of  illumina- 
tion. ^ 

The  more  we  extend  our  knowledge  of  the  operations  of 
creative  power,  as  manifested  in  the  structure  and  economy 
of  organized  beings,  the  better  we  become  qualified  to  ap- 
preciate the  intentions  with  which  the  several  arrangements 
and  constructions  have  been  devised,  the  aKt  with  which  they 
have  been  accomplished,  and  the  grand  comprehensive  plan 
of  which  they  form  a  part.  By  knowing  the  general  ten- 
dencies of  analogous  formations,  we  can  sometimes  recognise 
designs  that  are  but  faintly  indicated,  and  trace  the  links 
which  connect  them  with  more  general  laws.  By  render- 
ing ourselves  familiar  with  the  handwriting  where  the  cha- 
racters are  clearly  legible,  we  gradually  learn  to  decipher 
the  more  obscure  passages,  and  are  enabled  to  follow  the 
continuity  of  the  narrative  through  chapters  that  would  other- 
wise appear  mutilated  and  defaced.  Hence,  the  utility  of 
comprehending  in  our  studies  the  whole  range  of  the  or- 
ganized creation,  with  a  view  to  the  discovery  of  final 
causes,  and  obtaining  adequate  ideas  of  the  power,  the  wis- 
dom, and  the  goodness  of  G  od. 


(     39     ) 


CHAPTER  II. 


THE  FUNCTIONS  OF  LIFE. 

The  intentions  of  the  Deity  in  the  creation  of  the  animal, 
kingdom,  as  far  as  we  are  competent  to  discern  or  compre- 
hend them,  are  referrible  to  the  following  classes  of  objects. 
The  first  relates  to  the  individual  welfare  of  the  animal,  em- 
bracing the  whole  sphere  of  its  sensitive  existence,  and  the 
means  of  maintaining  the  vitality  upon  which  that  existence 
is  dependent.  The  second  comprises  the  provisions  that  have 
been  made  for  repairing  the  chasms  resulting,  in  the  present 
circumstances  of  the  globe,  from  the  continual  destruction 
of  life,  by  ensuring  the  multiplication  of  the  species,  and  the 
continuity  of  the  race  to  which  each  animal  belongs.  The 
the  third  includes  all  those  arrangements  which  have  been 
resorted  to,  in  order  to  accommodate  the  system  to  the  con- 
sequences that  follow  from  an  indefinite  increase  in  the 
numbers  of  each  species.  The  fourth  class  relates  to  tliat 
systematic  economy  in  the  plans  of  organization  by  which 
all  the  former  objects  are  most  effectually  secured.  I  shall 
offer  some  observations  on  each  of  these  general  heads  of  in- 
quiry. 

With  reference  to  the  welfare  of  the  individual  animal, 
it  is  evident  that  in  the  brute  creation,  the  great  end  to  be 
answered  is  the  attainment  of  sensitive  enjoyment.  To  this 
all  the  arrangements  of  the  system,  and  all  the  energies  of 
its  vital  powers  must  ultimately  tend.  Of  what  value  would 
be  mere  vegetative  life  to  the  being  in  whom  it  resides,  un- 
less it  were  accompanied  by  the  faculty  of  sensation,  and 
unless  the  sensations  thence  arising  were  attended  with  plea- 
sure?   It  is  only  by  reasoning  analogically  from  the  feelings 


40  THE  FUNCTIONS  OP  LIFE. 

we  have  ourselves  experienced  that  we  ascribe  similar  feel- 
ings to  other  sentient  beings,  and  that  we  infer  their  existence 
from  the  phenomena  which  they  present.  Wherever  these 
indications  of  feeling  are  most  distinct,  we  find  that  they 
result  from  a  particular  organization,  and  from  the  affections 
of  a  peculiar  part  of  that  organization  denominated  the  ner- 
vous substance.  The  name  of  brain  is  given  to  a  particular 
mass  of  this  substance  placed  in  the  interior  of  the  body, 
where  it  is  carefully  protected  from  injury. 

The  sensations,  for  exciting  which  the  brain  is  the  mate- 
rial instrument,  or  immediate  organ,  are  the  result  of  cer- 
tain impressions  made  on  particular  parts  of  the  body,  and 
conveyed  to  that  organ  by  the  medium  of  filaments,  com- 
posed of  a  similar  substance,  and  termed  nerves.  In  this 
way,  then,  it  has  been  provided  that  a  communication  shall 
be  established  between  the  sentient  principle  and  the  ex- 
ternal objects,  by  which  its  activity  is  to  be  excited,  and  on 
which  it  is  to  be  dependent  for  the  elements  of  all  its  af- 
fections, both  of  sensation  and  of  intellect.  A  considerable 
portion  of  this  treatise  will  be  occupied  with  the  develop- 
ment of  the  series  of  means  by  which  impressions  from  ex- 
ternal objects  are  made  on  the  appropriate  organs  that  are 
provided  to  receive  and  collect  them,  so  as  not  only  to  give 
rise  to  varied  sensations,  but  also  to  convey  a  knowledge  of 
the  existence  and  different  qualities  of  the  objects  that  pro- 
duce them.     This  latter  faculty  is  termed  Perception. 

But  in  the  formation  of  animals  it  was  not  the  intention 
of  Providence  to  endow  them  with  the  mere  capacity  of 
being  affected  by  surrounding  objects,  and  of  deriving  from 
them  various  sensations  of  pleasure  and  of  pain,  without 
granting  them  the  power  of  controlling  these  effects,  and  of 
acting  on  those  objects  in  return.  The  faculties  of  sensation 
and  perception,  in  beings  destined  to  be  merely  passive, 
and  the  sport  of  every  contingent  agency,  would  have  been 
not  merely  useless,  but  even  baneful  endowments.  The  same 
beneficent  power  which  has  conferred  these  gifts  has  con- 
joined that  of  voluntary  motion,  by  which  the  animal  may 
not  only  obtain  possession  of  such  objects  as  minister  to  its 


THE  FUNCTIONS  OP  LIFE.  41 

gratification,  and  reject  those  which  are  useless  or  hurtful, 
but  may  always  move  from  place  to  place,  and  enlarge  the 
sphere  of  its  perceptions  and  of  its  power.  The  same  mass 
of  nervous  substance  which,  under  the  name  of  brain,  we 
have  recognised  as  the  organ  of  sensation,  is  also,  as  will  af- 
terwards be  shown,  the  organ  of  volition;  and  the  medium, 
by  which  the  commands  of  the  will  are  transmitted  from 
the  brain  to  the  mechanical  apparatus  employed  for  motion, 
is  again  certain  filaments  of  nerves;  but  these  nervous  fila- 
ments are  distinct  from  those  which  are  subservient  to  sen- 
sation. 

Next  in  importance,  then,  to  the  organs  of  sensation  and 
perception,  are  those  of  Voluntary  Motion.  They  com- 
prise two  kinds  of  objects;  first,  the  establishment  of  a  cer- 
tain mechanism  having  the  cohesion,  the  strength,  and  the 
mobility  requisite  for  the  different  actions  which  the  animal 
is  to  perform;  and,  secondly,  the  provision  of  a  power,  or 
agent,  which  shall  be  capable  of  supplying  the  mechanical 
force  for  setting  this  machinery  in  motion.  With  these  ob- 
jects must  be  combined  various  subsidiary  arrangements  re- 
lating to  the  connexions,  the  support,  the  protection,  and 
other  mechanical  conditions  of  the  organs  of  the  body.  It  will 
be  convenient  to  comprehend  these  under  one  general  head, 
considering  them  as  composing  the  Mechanical  Functions 
of  the  animal  economy.  They  will  engage  a  considerable 
share  of  our  attention  in  this  work,  as  afibrding  the  clearest 
and  most  palpable  proofs  of  contrivance  and  design. 

From  the  peculiar  conditions  of  the  living  body,  not  only 
with  regard  to  the  mechanical  properties  of  its  various  parts, 
and  the  powers  by  which  their  movements  are  effected,  but 
also  with  regard  to  the  chemical  laws  which  regulate  the 
combinations  of  elements  composing  the  substance  of  the 
body,  there  is  required,  as  will  be  more  fully  explained  in 
the  sequel,  a  continual  renovation  of  that  substance.  For 
this  purpose  new  materials  are  perpetually  wanted,  and  must 
be  as  regularly  supplied.  Hence  arises  a  new  class  of  func- 
tions, comprising  a  great  extent  of  operations,  opening  a  wide 
Vol.  I.  6 


42  THE  FUNCTIONS  OF  LIFE. 

field  of  curious  and  interesting  inquiry,  and  furnishing  abun- 
dant evidence  of  the  wise  and  beneficent  operations  of  na- 
ture. These  may  be  comprehended  under  a  separate  class 
bearing  the  general  title  oi  JVutritivc  Functions.  They  are 
often  also,  spoken  of  under  the  designation  of  the  Vital 
Ftinctions,  from  their  more  immediate  relation  to  the  con- 
tinuance of  vitality;  that  is,  of  mere  vegetative  life,  as  dis- 
tinguished from  the  exercise  of  the  hidier  faculties  of  sen- 
sation,  perception,  and  voluntary  motion,  which  are  the  ul- 
timate ends  of  the  animal  existence,  and  which  are  empha- 
tically termed  the  Animal  Functions. 

The  vital  as  well  as  the  animal  functions  require  for  the 
execution  of  their  various  objects  certain  instruments  of  an 
appropriate  mechanical  construction,  adapted  to  those  objects. 
To  the  contrivances  of  the  mechanist  must  be  added  a  refined 
hydraulic  apparatus  for  the  conve3^ance  of  fluids,  and  for  the 
resrulation  of  their  movements;  and  with  these  must  be  con- 
joined  the  skilful  combinations  of  the  laborator}^,  by  which 
the  powers  of  the  most  subtle  chemistry  are  exercised  in  ef- 
fecting all  the  transmutations  required  by  this  elaborate  sys- 
tem of  operations.  As  far  as  they  involve  mechanical  prin- 
ciples, these  objects  again  arrange  themselves  under  the  me- 
chanical functions:  and  I  shall  accordingly  include  them 
under  that  head,  when  giving  an  account  of  this  branch  of 
the  subject. 

There  is  another,  and  a  most  important  consequence  that 
flows  from  the  peculiar  chemical  conditions  of  the  materials  of 
which  animal  structures  are  composed.  The  mode  in  which 
their  elements  are  combined  is  so  complex  as  to  require  a 
long  and  elaborate  process  to  accomplish  that  purpose;  and 
neither  tlie  organs  with  which  animals  are  furnished,  nor 
the  powers  with  which  those  organs  are  endowed,  are  ade- 
quate to  tlie  conversion  of  the  materials  furnished  by  the 
inorganic  world  into  the  substances  required  for  the  con- 
struction of  their  bodies,  and  the  maintenance  of  their 
powers.  These  inorganic  elements  must  have  passed 
through  intermediate  stages  of  combination,  and  must 
have    been   previously   elaborated   by  other  organized  be- 


THE  FUNCTIONS  OF  LIFE.  43 

ings.  This  important  office  is  consigned  to  the  vege- 
table kingdom.  Receiving  the  simple  food  furnished  by 
nature,  which  consists  chiefly  of  water,  air,  and  carbonic 
acid,  together  with  a  small  proportion  of  other  subslan  es, 
plants  convert  these  aliments  into  products,  which  not  only 
maintain  their  own  vitality,  but  serve  the  farther  purpose 
of  supporting  the  life  of  animals.  Thus  was  the  creation 
and  continuance  of  the  vegetable  kingdom  a  necessary  step 
towards  the  existence  of  the  animal  world;  as  well  as  a  link 
in  the  great  chain  of  being,  formed  and  sustained  by  Al- 
mighty power.  The  physiology  of  Vegetables  presents 
many  topics  of  great  interest  with  relation  to  final  causes, 
and  will  in  this  Treatise  be  reviewed  with  special  reference 
to  this  important  object. 

Nutrition,  both  in  the  vegetable  and  animal  sj'stems,  com- 
prises a  very  extended  scries  of  operations.     In  the  former 
it  includes  the  absorption  of  the  crude  materials  from  the 
surrounding  elements, — their  transmission  to  organs  where 
they  are  aerated,  that  is,  subjected  to  the  chemical  action  of 
the  air; — their  circulation  in  the  different  parts  of  the  plant, 
— their  farther  elaboration  in  particular  vessels  and  recep- 
tacles—their deposition  of  solid  materials — and  their  con- 
version into  peculiar  products,  as  well  as  into  the  substances 
which  compose  the  several  organs; — and,  finally,  the  growth 
and  development  of  the  whole  plant.     Still  more  various 
and  complicated  are  the  corresponding  functions  in  animals. 
Their  objects  may  be  arranged  under  the  following  general 
heads;  each,  again,  admitting  of  farther  subdivision.     The 
first  end  to  be  accomplished  is  to  animalize  the  food;  that 
is,  to  convert  it  into  a  matter  having  the  chemical  properties 
of  the  animal  substances  with  which   it  is  to  be  afterwards 
incorporated.     The  entire   change  thus  effected  is  termed 
Jissimilation^  of  which  Digestion  forms  a  principal  part. 
The  second  object  is  to  collect  and  distribute  this  prepared 
nutriment,  which  is  the  blood,  to  the  different  organs,  or 
wherever  it  may  be  wanted.      The  necessary  motions  for 
these  purposes  are  given  to  the  blood  by  the  organs  of  Cir- 
culation, consisting  of  the  Heart,  which  impels  it  /hrough 


44  THE  FUNCTIONS  OP  LIFE. 

a  system  of  pipes  called  Jirteries,  and  receives  it  back  again 
by  means  of  another  set  of  tubes  called  Veins,  In  the  third 
place  it  is  necessary  that  the  circulating  blood  should  con- 
tinually undergo  purification  by  the  chemical  action  of  oxy- 
gen: a  purpose  which  is  answered  by  the  function  of  Res- 
piratloii.  The  fourth  stage  of  nutrition  relates  to  the  more 
immediate  application  of  this  purified  material  to  the  wants 
of  the  S3^stem,  to  the  extention  of  the  organs,  to  the  repara- 
tion of  their  losses,  and  to  the  restoration  of  their  exhausted 
powers. 

Life,  then,  consists  of  a  continued  series  of  actions  and  re- 
actions, ever  varying,  yet  constantly  tending  to  definite  ends. 
Most  of  the  parts  of  which  the  body  consists  undergo  con- 
tinual and  progressive  changes  in  their  dimensions,  figure, 
arrangement,  and  composition.  The  materials  which  have 
been  united  together  and  fashioned  into  the  several  organs, 
are  them.selves  successively  removed  and  replaced  by  others, 
which  again  are,  in  their  turn,  discarded,  and  new  materials 
substituted,  though  without  any  perceptible  change  of  ex- 
ternal form.  Perpetual  mutation  appears  to  constitute  the 
fundamental  law  of  living  nature;  and  it  has  been  farther 
decreed  by  the  power  which  gave  the  first  impulse  of  ani- 
mation to  this  organized  fabric,  that  its  movements  and  its 
powers  shall  be  limited  in  their  duration,  and  that,  even 
when  they  are  not  destroyed  by  extraneous  causes,  after 
continuing  for  a  certain  period,  they  shall  come  to  a  close. 
The  law  of  Mortality,  to  which  all  the  beings  that  have  re- 
ceived the  gift  of  life  are  subjected,  is  a  necessary  conse- 
quence of  the  law  of  mutation;  and  the  same  causes  that 
originally  effected  the  development  and  growth  of  the  sys- 
tem, and  maintained  it  in  the  vigour  of  its  maturity,  by  con- 
tinuing to  operate,  are  certain  to  lead  to  the  demolition  of 
the  fabric  they  had  raised,  and  to  the  exhaustion  and  final 
extinction  of  its  powers.  The  individual  dies;  but  it  is  only 
to  give  place  to  other  beings,  alike  in  nature  and  in  form, 
equally  partaking  of  the  blessings  of  existence,  and  destined, 
after  having,  in  their  turn,  given  rise  to  a  new  race  of  sue- 


THE  FUNCTIONS  OF  LIFE.  45 

cessors,  to  run  through  the  same  perpetual  cycle  of  changes 
and  renovations. 

Thus  the  continuance  and  multiplication  of  each  species 
may  be  assigned  as  the  second  of  the  great  ends  which  are 
to  be  accomplished  in  the  system  of  living  nature.     A  por- 
tion of  the  vital  power  of  the  parent  Is  for  this  purpose  em- 
ployed to  give  origin  and  birth  to  the  offspring.     The  pro- 
cess itself,  by  which  the  germs  of  living  beings  originate,  Is 
veiled  in  the  most  Impenetrable  mystery.     But  we  are  per- 
mitted to  trace  many  of  the  subsequent  steps  In  the  gradual 
development  both  of  vegetable  and  animal    organizations; 
and  certainly  no  part  of  the  economy  of  animated  nature  is 
more  calculated  to  impress  us  with  exalted  ideas  of  the  im- 
mensity of  the  scheme  of  Providence,  and  the  vigilant  care 
with  which  the  most  distant  consequences  have  been  antici- 
pated, than  the  history  of  the  early  periods  of  their  existence. 
Nothing  can  be  more  admirable  than  the  progressive  archi- 
tecture of  the  frame;  nothing  more  beautiful  than  the  setting 
up  of  temporary  structures,  which  are  required  only  at  an 
early  stage  of  growth,  and  which  are  afterwards  removed  to 
give  place  to  more  permanent  and  finished  organs. 

The  utmost  solicitude  has  been  shown  in  every  part  of 
living  nature  to  secure  the  perpetuity  of  the  race,  by  the 
establishment  of  laws,  of  which  the  operation  is  certain  In 
all  contingent  circumstances.  It  has  also  been  manifestly 
the  object  of  various  provisions  to  diffuse  the  races  as  widely 
as  possible  over  a  great  surface  of  the  habitable  globe. 

We  are  next  to  advert  to  the  Important  consequences 
which,  in  the  animal  kingdom  more  especially,  flow  from 
this  law  of  indefinite  production.  As  animals  are  ultimately 
dependent  on  the  vegetable  kingdom  for  the  materials  of 
their  subsistence,  and  as  the  quantity  of  these  materials  is,  in 
a  state  of  nature,  necessarily  limited  by  the  extent  of  surface 
over  which  vegetation  is  spread,  a  time  must  arrive  when 
the  number  of  animals  thus  continually  Increasing  Is  exactly 
such  as  the  amount  of  food  produced  by  the  earth  will  main- 
tain.     When  this  limit  has   been  attained,  no  farther  in- 


46  THE  FUNCTIONS  OF  LIFE. 

crease  can  take  place  in  their  number,  except  by  resorting 
to  the  expedient  which  we  find  actually  adopted,  namely, 
that  of  employing  the  substance  of  one  animal  for  the  nourish- 
ment of  others.  Thus  the  identical  combinations  of  ele- 
ments, effected  by  the  powers  of  vegetation,  are  transformed 
in  succession  from  one  living  being  into  another,  and  be- 
come subservient  to  the  maintenance  of  a  great  number  of 
different  animals  before  they  finally,  by  the  process  of  de- 
composition, revert  to  their  original  inorganic  state. 

**  See  (lying"  vegetables  life  sustain, 
See  life  dissolving-  veg-etate  again; 
All  forms  that  perish  other  forms  suppl)'-. 
By  turns  we  catch  the  vital  breath  and  die." — Pope. 

Hence  has  the  ordinance  been  issued  to  a  large  portion  of 
the  animal  world  that  they  are  to  maintain  themselves  by 
preying  upon  other  animals,  either  consuming  their  substance 
when  already  dead,  or  depriving  them  of  life  in  order  to  pro- 
Ions:  their  own.  Such  is  the  command  given  to  the  count- 
less  hosts  of  living  beings  which  people  the  vast  expanse  of 
ocean;  to  the  unnumbered  tribes  of  insects  which  every  spot 
of  earth  discloses;  to  the  greater  number  of  the  feathered 
race;  and  also  to  a  more  restricted  order  of  terrestrial  ani- 
mals. To  many  has  the  commission  been  given  to  ravage 
and  to  slaughter  by  open  violence;  others  are  taught  more 
insidious,  though  no  less  certain  arts  of  destruction;  and  some 
appear  to  be  created  chiefly  for  the  purpose  of  quickly  clear- 
ing the  earth  of  all  decomposing  animal  or  vegetable  mate- 
rials, which  might  otherwise  have  filled  the  air  with  noxious 
exhalations  and  contaminated  the  sources  of  vitality.* 

This  new  law  of  animal  existence  must  necessarily  intro- 
duce new  conditions  of  organization  and  of  functions.  Struc- 
tures adapted  to  rapid  locomotion  must  be  supplied  for  the 

*  As  especially  appointed  for  the  performance  of  this  useful  task  may  be 
cited,  among-  the  larger  beasts  of  prey,  the  hyaena,  the  jackal,  the  crow,  and 
the  vulture:  among- marine  animals,  the  Crustacea,  and  mimerous  mollusca; 
and  among  the  lower  orders,  imiumerable  tribes  of  insects,  such  as  ants, 
flesh  flies,  he. 


THE  FUNCTIONS  OF  LIFE.  47 

pursuit  of  prey,  and  powerful  weapons  for  attack  and  de- 
struction. But  nature  has  not  left  the  weaker  animals  un- 
provided with  the  means  of  repulse,  of  defence,  or  of  escape. 
For  these  purposes  various  expedients,  either  of  force,  of 
swiftness,  or  of  stratagem,  have  been  resorted  to  in  different 
cases. 

That  a  large  portion  of  evil  is  the  direct  consequence  of 
this  system  of  extensive  warfare,  it  is  in  vain  to  deny.  But 
although  our  sensibility  may  revolt  at  the  wide  scene  of 
carnage  which  is  so  generally  presented  to  our  view,  our 
more  sober  judgment  should  place  in  the  other  scale  the  great 
preponderating  amount  of  gratification  which  is  also  its  re- 
sult. We  must  take  into  account  the  vast  accession  that 
accrues  to  the  mass  of  animal  enjoyment  from  the  exercise 
of  those  powers  and  faculties  which  are  called  forth  by  this 
state  of  constant  activity;  and  when  this  consideration  is 
combined,  as  it  ought  to  be,  with  that  of  the  immense  multi- 
plication of  life  which  is  admissible  upon  this  system  alone, 
we  shall  find  ample  reason  for  acknowledging  the  wisdom 
and  the  benevolent  intentions  of  the  Creator,  who,  for  the 
sake  of  a  vastly  superior  good,  has  permitted  the  existence 
of  a  minor  evil. 

From  this  system  of  hostilities  there  must  also  arise  new 
relations  among  the  different  races  of  animals.  It  affords  a 
ready  and  effectual  means  of  preserving  the  proper  balance 
between  different  races.  Each  separate  species  of  animals, 
far  from  being  isolated  and  independent,  performs  the  part 
assigned  to  it  in  the  system  of  nature,  and,  however  appa- 
rently insignificant,  may  have  a  sensible  influence  on  the 
rest  of  the  animal  creation.  Man,  above  all  other  animals, 
has  effected  a  most  important  change  in  the  condition  of  the 
multitude  of  other  races,  in  every  region  where  his  numbers 
have  multiplied,  where  the  arts  of  civilization  have  enlariied 
his  dominion,  and  where  science  has  armed  him  with  still 
more  extensive  power. 

In  every  department  of  nature  it  cannot  fail  to  strike  us 
that  boundless  variety  is  a  charactesistic  and  predominant 
feature  of  her  productions.     It  is  only  when  the  object  to 


48  THE  FUNCTIONS  OF  LIFE. 

be  attained  is  dependent  upon  certain  definite  conditions,  ex- 
cluding the  possibility  of  modification,  that  these  conditions 
are  uniformly  and  strictly  adhered  to.  But  wherever  that 
absolute  necessity  does  not  exist,  and  there  is  afibrded  scope 
for  deviation,  there  we  are  certain  to  find  introduced  all 
those  modifications  which  the  occasion  admits  of.  Not  only 
is  this  tendency  to  variety  exemplified  in  the  general  ap- 
pearance and  form  of  the  body,  but  it  also  prevails  in  each 
individual  organ,  however  minute  and  insignificant  that  or- 
gan may  seem.  Even  when  the  purpose  to  be  answered 
is  identical,  the  means  that  are  employed  are  infinitely  di- 
versified in  different  instances,  as  if  a  design  had  existed  of 
displaying  to  the  astonished  eyes  of  mortals  the  unbounded 
resources  of  creative  power.  While  the  elements  of  struc- 
ture are  the  same,  there  is  presented  to  us  in  succession 
every  possible  combination  of  organs,  as  if  it  had  been  the 
object  to  exhaust  all  the  admissible  permutations  in  the  order 
of  their  union. 

Some  wise  purpose,  though  dimly  perceptible  to  our  im- 
perfect understandings,  is  no  doubt  answered  by  this  great 
law  of  organic  formation,  the  law  of  variety.  That  it  is  not 
blindly  or  indiscriminately  followed,  is  apparent  from  its 
being  circumscribed  within  certain  limits,  and  controlled  by 
another  law,  w^hich  we  have  next  to  consider — that  of  con- 
formity to  a  definite  type. 

The  most  superficial  survey  of  nature  is  sufficient  to  show 
that  there  prevail  certain  general  resemblances  among  great 
multitudes  of  species,  which  lead  us  to  class  them  into  more 
or  less  comprehensive  groups.  Thus  in  the  animal  kingdom, 
quadrupeds,  birds,  fishes,  reptiles,  shell-fish,  and  insects, 
compose  natural  assemblages  or  classes,  and  each  of  these 
is  readily  divisible  into  subordinate  groups  or  families.  Now 
it  results  from  a  closer  examination  of  the  structure  and 
economy  of  plants  and  animals,  that  the  formation  of  all  the 
individual  species  comprehended  in  the  same  class,  has  been 
conducted  in  conformity  with  a  certain  ideal  model,  or  type, 
as  it  is  called.  Of  this  general  type  all  the  existing  forms 
appear  as  so  many  separate  copies,  differing,  indeed,  as  to 


THE  FUNCTIONS  OF  LIFE.  49 

particulars,  but  agreeing  as  to  general  characters.  The  same 
observation  applies  to  the  families,  the  genera,  and  other 
subordinate  groups  of  living  beings. 

The  more  extensive  our  acquaintance  is  with  the  anatomy 
and  physiology  of  both  plants  and  animals,  the  more  striking 
do  these  analogies  appear;  so  that  amidst  endless  diversity 
in  the  details  of  structures  and  of  processes,  the  same  general 
purpose  is  usually  accomplished  by  similar  organs  and  in 
similar  modes.  So  firmly  is  this  principle  established,  that 
we  may  venture  with  confidence  to  predict  many  circum- 
stances relating  to  an  unknown  animal,  of  which  only  a  few 
fragments  are  presented  to  us,  from  our  general  knowledge 
of  the  characters  and  economy  of  the  tribe  or  family,  on  the 
type  of  which  it  has  been  modelled.  Thus,  the  discovery 
of  a  mutilated  portion  of  the  skeleton  of  a  fossil  animal,  gives 
to  the  physiologist,  who  is  conversant  with  the  details  of 
comparative  anatomy,  a  knowledge  of  the  general  structure 
and  habits  of  that  animal,  though  all  other  traces  of  its  exist- 
ence may  have  been  swept  away,  amidst  the  primeval  revo- 
lutions of  the  globe.* 

Not  only  does  this  tendency  to  conform  to  particular  types 
obtain  in  all  organic  formations,  but  farther  inquiry  leads  to 
the  conclusion  that  the  deviations  from  these  standard  forms, 
far  from  being  arbitrary,  are  themselves  referrible  to  par- 
ticular laws.  The  regulating  principle  of  the  variations  is 
subordinate  to  higher  views,  and  has  reference  to  tlie  respec- 
tive objects  and  destination  of  each  particular  species  in  the 
general  system  of  created  beings.  Nature,  as  far  as  we  can 
discern,  appears,  in  conformity  with  these  intentions,  first 
to  have  laid  down  certain  great  plans  of  functions  to  which 
she  has  adapted  the  structure  of  the  organs;  the  minor  ob- 
jects and  more  subordinate  functions  being  accommodated 
to  this  general  design.  Hence  arises  the  necessary  and  re- 
ciprocal dependence  of  each  organ  and  of  each  function  on 

*  See  Cuvier's  "Discours  sur  les  Revolutions  de  la  Surface  du  Globe,*'  p. 
47,  prefixed  to  the  first  volume  of  lii;^  Osscmens  Fossiies.'* 

Vol.  I.  7 


50  THE  FUNCTIONS  OF  LIFE. 

every  other;  and  hence  are  deduced  what  have  been  termed 
the  laios  of  the  co-existence  of  organic  forms.  By  attention 
to  these  laws  we  may  often  explain  how  each  variation  that 
is  observed  in  any  one  organ,  common  to  a  natural  group 
of  animals,  entails  certain  necessary  and  corresponding  va- 
riations in  other  parts,  and  extends  its  influence  in  modify- 
ing, in  a  greater  or  less  degree,  the  whole  fabric.  It  is  in 
comparative  anatomy  as  in  mechanics,  where  any  alteration 
made  in  the  position  of  one  part  of  a  system  of  bodies  occa- 
sions a  change  in  the  centres  of  gravity,  of  gyration,  and  of 
oscillation;  and  evolves  new  mechanical  forces  and  condi- 
tions of  equilibrium,  which  render  new  adjustments  in  other 
parts  necessary,  in  order  to  restore  the  equipoise,  and  pre- 
serve the  harmony  of  their  movements. 

We  may  conclude  from  these  inquiries  that  the  numerous 
classes  or  assemblages  of  beings,  which  science  has  formed, 
are  by  no  means  arbitrary  creations  of  the  human  mind,  in- 
vented merely  with  a  view  to  facilitate  the  study  and  to 
recognise  the  identity  of  species,  or  calculated  only  to  sup- 
ply the  imperfections  of  our  memory;  but  that  they  have  a 
real  foundation  in  nature.  To  regard  any  of  the  beings  in 
the  creation  as  isolated  from  the  rest,  would  be  to  take  a 
very  narrow  and  a  false  view  of  their  condition;  for  all  are 
connected  by  mutual  relations.  Even  among  the  leading 
types  which  represent  the  great  divisions  of  the  animal  king- 
dom we  may  trace  several  points  of  resemblance,  which  show 
them  to  be  parts  of  one  general  plan,  and  to  have  emanated 
from  the  same  Creator.  In  the  progress  of  discovery  we 
are  continually  meeting  with  species  which  occupy  interme- 
diate places  between  adjacent  types,  and  appear  as  links  of 
connexion  in  the  chain  of  being.  It  often  happens,  as  I 
shall  hereafter  have  occasion  to  point  out,  that  throughout 
an  extensive  series  of  organic  forms,  the  steps  of  gradation 
by  which  one  type  passes  into  another,  are  so  numerous  and 
so  regular,  as  to  preclude  the  possibility  of  drawing  a  de- 
cided line  of  demarcation  between  those  that  properly  ap- 
pertain to  each.  " 


THE  FUNCTIONS  OF  LIFE.  51 

All  these  apparent  anomalies  and  gradations  of  structure 
tend  still  farther  to  demonstrate  the  generality  of  the  plans 
of  nature,  and  the  comprehensiveness  of  her  design,  which 
embraces  the  whole  series  of  animated  beings.  These  views 
are  strongly  corroborated  by  the  discoveries  that  are  con- 
tinually being  made  of  species  now  no  longer  in  existence, 
but  which,  in  former  ages  of  the  world,  helped  to  fill  up 
many  of  the  chasms  which  now  interrupt  the  continuity  of 
that  series.  This  knowledge  has  been  revealed  to  us  by  the 
examination  of  their  fossil  remains,  those  monuments  of 
former  epochs,  which  have  thrown  such  important  light  on 
the  most  interesting  questions  in  Geology  as  well  as  in  Phy- 
siology.       ^ 

The  notion  has  long  prevailed  that  the  beings  composing 
the  vegetable  and  animal  kingdoms,  might,  if  we  were 
thoroughly  acquainted  with  their  structure  and  economy,  be 
arranged  in  a  linear  series,  commencing  with  the  simplest 
and  regularly  ascending  to  the  most  refined  and  complicated 
organizations,  till  it  reached  its  highest  point  in  man,  who  is 
unquestionably  placed  at  the  summit  of  the  scale.  Bonnet, 
in-particular,  cherished  with  enthusiastic  ardour  the  hypo- 
thesis that  all  organic  beings  formed  a  continuous  gradation, 
each  member  of  which,  like  the  successive  links  of  a  chain, 
was  connected  with  that  which  preceded,  and  with  that 
which  followed  it;  and  he  pursued  this  idea  by  appljdng  it 
even  to  the  productions  of  the  mineral  world.  But,  divest- 
ing ourselves  of  these  hypothetical  views  and  figurative 
images,  we  find,  on  sober  observation,  that  instead  of  one 
continuous  series,  we  are  presented  with  only  detached  frag- 
ments and  interrupted  portions  of  this  imaginary  system:  so 
that,  if,  for  the  sake  of  illustration,  we  must  employ  a  me- 
taphor, the  natural  distribution  of  animals  would  appear  to 
be  represented,  not  by  a  chain,  but  by  complicated  net- 
work, where  several  parallel  series  are  joined  by  transverse 
and  oblique  lines  of  connexion.  A  multitude  of  facts,  how- 
ever, tend  to  show  that  the  real  types  or  models  of  struc- 
ture, are  more  correctly  represented  by  circular  or  recurring 


52  THE  FUNCTIONS  OF  LIFE. 

arransements.*  But  as  the  discussion  of  these  and  other 
topics  relating  to  the  plans  and  designs  of  nature  in  the  for- 
mation of  organic  beings  requires  a  previous  acquaintance 
with  the  details  of  comparative  anatomy  and  physiology,  I 
shall  defer  all  farther  observations  respecting  them  till  I 
have  finished  the  review  T  propose  to  take  of  the  several 
structures  and  functions  of  the  animal  and  vegetable  econo- 
my. There  are,  however,  some  views  that  have  been  en- 
tertained respecting  the  procedure  of  nature  in  the  formation 
of  the  different  races  of  animals,  which  it  will  be  proper  to 
notice  in  this  place,  as  they  will  occasionally  be  referred  to 
when  the  facts  that  more  particularly  illustrate  and  support 
them  come  to  be  noticed.  -^ 

An  hypothesis  has  been  advanced  that  the  original  crea- 
tion of  species  has  been  successive,  and  took  place  in  the  or- 
der of  their  relative  complexity  of  structure;  that  the  stan- 
dard types  have  arisen  the  one  from  the  other;  that  each 
succeed  in  g  form  was  an  improvement  upon  the  preceding, 
and  followed  in  a  certain  order  of  development,  according 
to  a  regular  plan  traced  by  the  great  Author  of  the  universe 
for  bestowing  perfection  on  his  works.  This  gradation-  of 
structure  was  necessarily  accompanied  by  a  gradation  of  fa- 
culties: the  object  of  each  change  of  type  being  to  attain 
higher  objects,  and  to  advance  a  farther  step  towards  the  ul- 
timate ends  of  the  animal  creation.  Many  apparent  anoma- 
lies which  are  inexplicable  upon  any  other  supposition,  are 
easily  reconcileable  to  this  theory.  The  developments  of 
structure  belonging  to  a  particular  type  being  always  pro-  • 
spective,  are  not  completed  in  the  inferior  orders  of  the 
group  formed  upon  that  model,  but  remain  more  or  less 
imperfect,  although  each  organ  always  fully  answers  the  par- 
ticular purpose  of  the  individual  animal.  But  it  sometimes 
happens  that  the  imperfection  of  an  organ  is  so  great,  in  con- 
sequence of  its  development  having  proceeded  to  a  very  small 

*  Mr.  M'Leay  is  the  author  of  this  ing-enious  theory,  which  he  has  de- 
veloped in  his  "  Horx  Entomologlca;"  and  which  appears  to  be  verified  to  a 
great  extent  by  the  modern  discoveries  in  comparative  anatomy. 


THE  FUNCTIONS  OF  LIFE.  5SL 

extent,  as  to  render  it  wholly  useless  in  that  particular  spe- 
cies, although  in  a  higher  race  of  animals  it  fully  performs 
its  proper  function.  Thus,  we  shall  find  that  rudiments  of 
feet  are  contained  within  the  bodies  of  various  kinds  of  ser- 
pents, which  can  obviously  not  be  serviceable  as  organs  of 
progression.  In  the  young  of  the  whale,  before  its  birth, 
there  is  found  in  the  lower  jaw,  a  row  of  small  teeth,  which 
do  not  rise  above  the  gums,  and  can,  therefore,  be  of  no  use 
as  instruments  of  mastication.  Their  farther  growth  is  ar- 
rested, and  they  are  afterwards  obliterated.  This  imperfect 
or  rudimentary  condition  of  an  organ  indicates  its  relation 
to  other  species  belonging  to  the  same  type,  and  demon- 
strates the  existence  of  a  general  plan  in  their  formation.  I 
shall  have  occasion  to  mention  several  striking  instances  of 
this  kind,  both  in  the  animal  and  vegetable  kingdom. 

In  followino;  the  transitions  from  one  model  of  structure 
to  another,  we  often  observe  that  a  particular  organ  has  been 
very  greatly  enlarged,  or  otherwise  modified  to  suit  some 
particular  purpose,  foreign  to  its  usual  destination,  or  to  qua- 
lify it  for  performing  some  new  office,  rendered  necessary 
by  the  particular  circumstances  in  which  the  animal  is 
placed.  Thus,  the  ribs,  which  in  quadrupeds  are  usually 
employed  for  respiration,  are  in  serpents  converted  into  aux- 
iliary organs  of  progressive  motion:  and  in  the  Draco  vo- 
lans,  or  flying  lizard,  they  are  extended  outwards  from  the 
sides  to  serve  as  wings.  The  teeth,  usually  intended  for 
mastication,  are  in  many  animals  enlarged  in  order  to  serve 
as  weapons  of  offence,  as  in  the  Elephant,  the  Boar,  the 
Narwal  and  the  Pristis.  In  like  manner,  in  the  Crustacea, 
organs  of  the  same  general  structure  are  converted  some- 
times into  jaws,  sometimes  into  feelers,  (or  palpi,)  and  some- 
times into  feet;  and  the  transition  from  the  one  to  the  other 
is  so  gradual  that  it  is  difficult  to  draw  a  proper  distinction 
between  them. 

In  pursuing  the  ascending  series  of  animal  structures  we 
meet  also  with  instances  of  a  contrary  change,  yet  still  re- 
sulting from  the  continued  application  of  the  same  principle. 


54  THE  FUNCTIONS  OF  LIFE. 

f 

An  organ  which  has  served  an  important  purpose  in  one 
animal,  may  be  of  less  use  in  another,  occupying  a  higher 
station  in  the  scale,  and  the  change  of  circumstances  may 
even  render  it  wholly  useless.  In  such  cases  we  find  that 
it  is  gradually  discarded  from  the  system,  becoming  conti- 
nually smaller,  till  it  disappears  altogether.  We  may  often, 
however,  perceive  some  traces  of  its  existence,  but  only  in 
a  rudimental  state,  and  a^  if  ready  to  be  developed,  when 
the  occasion  may  demand  it. 

In  the  greater  number  of  organic  structures  we  may  trace 
a  tendency  to  the  repetition  of  certain  organs,  or  parts,  and 
the  regular  arrangement  of  these  similar  portions  either 
round  a  central  axis,  or  in  a  longitudinal  series.  The  for- 
mer is  apparent  in  the  verticillated  organs  of  plants,  and  in 
the  radiated  forms  of  zoophytes.  The  linear  arrangement 
is  exhibited  in  the  similar  segments  of  annulose  and  other 
articulated  animals,  and  also  in  the  pieces  which  compose 
the  spinal  column  of  vertebrated  animals.  In  these  two 
latter  classes,  also,  a  remarkable  law  of  symmetry  obtains 
in  the  formation  of  the  two  sides  of  the  body,  which  ex- 
hibits the  lateral  junction  of  similar  but  reversed  structures. 
The  violations  of  this  law  are  extremely  rare;  yet  some  re- 
markable instances  of  anomalous  formations,  in  this  respect, 
will  hereafter  be  noticed. 

In  treating  of  the  particular  functions  of  the  animal  and 
vegetable  economy  I  shall  follow  a  different  order  from  that 
in  which  I  have  presented  them  in  the  preceding  sketch. 
As  the  Mechanical  functions  depend  upon  the  simpler  pro-  " 
perties  of  matter  and  the  well  known  laws  of  Mechanism,  I 
think  it  best  to  commence  with  the  examination  of  these. 
Our  attention  will  next  be  directed  to  the  highly  interesting 
subjects  which  relate  to  the  Nutritive  or  Vital  functions 
both  of  vegetable  and  animal  structures:  for  as  they  involve 
the  chemical  properties  of  organized  substances,  and  are, 
therefore,  of  a  more  refined  and  intricate  nature  than  the 
preceding,  I  conceive  they  will  be  best  understood  after  the 
general  mechanism  of  the  frame  has  been  explained.   These 


THE  FUNCTIONS  OF  LIFE.  55 

Studies  will  prepare  us  for  the  consideration  of  living  animals 
as  sentient  and  active  beings,  endowed.by  their  bounteous 
Creator  with  the  exalted  faculties  of  perception  and  of  vo- 
lition, which  alone  give  value  to  existence,  and  which  raise 
them  so  far  above  the  level  of  the  vegetable  world.  I  shall 
lastly  give  a  very  brief  account  of  the  reproductive  func- 
tions and  of  the  phenomena  of  animal  development,  in  which 
the  discoveries  of  modern  times  have  revealed  to  us  so  con- 
siderable a  portion  of  those  extensive  plans  which  an  all-wise 
Providence  has  beneficently  devised  for  the  general  wel- 
fare of  animated  beings. 


PART    1. 


THE  MECHANICAL  FUNCTIONS. 


CHAPTER  I. 


ORGANIC  MECHANISM. 

§.  1.   Organization  in  General. 

Life,  which  consists  of  a  continued  series  of  actions,  di- 
rected to  particular  purposes,  cannot  be  carried  on  but  by 
the  instrumentality  of  those  peculiar  and  elaborate  structures 
and  combinations  of  material  particles  which  constitute  or- 
ganization. All  these  arrangements,  both  as  respects  the 
mechanical  configuration  and  the  chemical  constitution  of 
the  elements  of  which  the  organized  body  is  composed, 
even  when  apparently  most  simple,  are,  in  reality,  complex 
and  artificial  in  the  highest  possible  degree.  Let  us  take  as 
a  specimen  the  crystalline  lens,  or  hard  central  part,  of  the 
eye  of  a  codfish,  which  is  a  perfectly  transparent,  and  to  all 
appearance  homogeneous,  spherule.  No  one,  unaccustomed 
to  explore  the  wonders  of  nature,  would  suspect  that  so 
simple  a  body,  which  he  might  suppose  to  be  formed  of  a 
uniform  material  cast  in  a  mould,  would  disclose,  when  ex- 
amined under  a  powerful  microscope,  and  with  the  skill  of 
a  Brewster,  the  most  refined  and  exquisite  conformation. 
Yet,  as  I  shall- have  occasion  to  specify  more  in  detail  in  its 


ORGANIC  MECHANISM.  57 

proper  place,  this  little  spherical  body,  scarcely  larger  than 
a  pea,  is  composed  of  upwards  of  five  millions  of  fibres,  which 
lock  into  one  another  by  means  of  more  than  sixty-two 
thousand  five  hundred  millions  of  teeth.  If  such  be  the 
complication  of  a  portion  only  of  the  eye  of  that  animal, 
how  intricate  must  be  the  structure  of  the  other  parts  of  the 
same  organ,  having  equally  important  offices!  What  ex- 
quisite elaboration  must  those  textures  have  received  whose 
functions  are  still  more  refined!  What  marvellous  w^ork- 
manship  must  have  been  exercised  in  the  organization  of  the 
nerves  and  of  the  brain,  those  subtle  instruments  of  the  hiirher 
animal  faculties,  and  of  which  even  the  modes  of  action  are 
to  us  not  merely  inscrutable,  but  surpassing  all  our  powers 
of  conception! 

It  is  from  the  energies  of  life  alone  that  organic  forms  are 
produced.  No  fabric  achieved  by  human  power  ever  ap- 
proached in  refinement  the  simplest  of  nature's  works.  The 
utmost  efforts  of  the  ingenuity  or  skill  of  man  in  the  con- 
struction of  the  most  delicate  machinery  is  infinitely  sur- 
passed by  the  most  ordinary  of  the  mechanisms  which  are 
presented  to  our  view  in  living  bodies.  However  success- 
ful may  be  human  artists  in  their  attempts  to  contrive  au- 
tomata, which  shall  exactly  imitate  different  animal  move- 
ments, there  will  always  be  wanting  that  internal  principle 
of  action  derived  from  a  higher  source  than  mechanism 
can  supply,  and  without  which  these  highly  wrought  works 
of  man,  like  the  unvivified  statues  of  Prometheus,  must  re- 
main for  evermore  masses  of  insentient  and  inert  materials. 

As  the  living  functions  imply  the  mechanical  action  and 
reaction  of  parts  which  cohere  in  some  definite  order  of 
arrangement  so  as  to  preserve  that  determinate  form  to  which 
they  constantly  tend  to  return  on  being  displaced,  it  is  im- 
possible to  conceive  that  a  mere  fluid  can  exercise  these 
functions;  because  the  particles  of  a  fluid,  being  equally 
moveable  in  every  direction,  have  no  determinate  relative 
situations,  and  possess  no  character  of  permanence.  All  or- 
ganic and  living  structures,  therefore,  must  be  composed  of 
solid  as  well  as  fluid  partsj  although  the_'proportion  between 

Vol.  I.  8  . 


58 


THE  MECHANICAL  FUNCTIONS. 


these  is,  in  different  cases,  almost  infinitely  varied.  A  dor- 
mant vitality  may,  indeed,  exist  in  a  system  of  organs  which 
have  been  brought  into  a  perfectly  dry  state:  as  is  proved 
by  the  examples  of  vegetable  seeds,  and  also  of  many  spe- 
cies of  animalcules,  and  even  of  some  of  the  more  highly 
developed  Jinnelida,  or  worms,  which  may  he  kept  in  a 
dry  state  for  an  indefinite  length  of  time,  and  when  moist- 
ened with  water,  resume  their  activity,  as  if  restored  to  life. 
The  germination  of  seeds  under  these  circumstances  is  matter 
of  common  observation;  but  the  revivification  of  animal- 
cules is  a  more  curious  phenomenon,  for  it  takes  place  more 
rapidly,  and  is  more  striking  in  its  results.  The  Rotifer 
redivivus,  or  wheel  animalcule,*  (Fig.  1,)  which  was  first 
observed  by  Lewenhoeck,  and  was  afterwards  rendered  ce- 
lebrated by  the  experiments  made  upon  it  by  Spallanzani.. 
can  live  only  in  water,  and  is  commonly  found  in  that  which 
has  remained  stagnant  for  some  time  in  the  gutters  of  houses. 
But  it  may  be  deprived  of  this  fluid,  and  reduced  to  perfect 


dryness,  so  that  all  the  functions  of  life  shall  be  completely 
suspended,  yet  without  the  destruction  of  the  vital  princi- 
ple; for  this  atom  of  dust,  after  remaining  for  years  in  a  dry 
state,  may  be  revived  in  a  few  minutes  by  being  again  sup- 
plied with  water.  This  alternate  suspension  and  restora- 
tion of  life  may  be  repeated,  without  apparent  injury  to  tlie 
animalcule,  for  a  great  number  of  times.  Similar  phenome- 
na are  presented  by  the  Vibrio  trilici,  (Fig.  2,)  or  the  ani- 


*  Vorlicella  rotatoria  of  Ginclin,  and  Furcularia  of  Laniuik. 


ORGANIC  MECHANISM.  59 

malcule  resemblino;  an  eel  in  its  shape,  which  infests  dis- 
eased wheat,  and  which,  when  dried,  appears  in  the  form 
of  a  fine  powder:  on  being  moistened  it  soon  resumes  its 
living  and  active  state.*  The  Gordius  aquaticiis,  or  hair 
worm,  which  inhabits  stagnant  pools,  and  which  remains  in 
a  dry  and  apparently  lifeless  state  when  the  pond  is  evapo- 
rated, will,  in  like  manner,  revive,  in  a  very  short  time,  on 
being  again  immersed  in  water.  The  same  phenomenon  is 
exhibited  by  the  Filaria,  a  thread-like  parasitic  worm,  in- 
festing the  cornea  of  the  eye  of  the  horse.t 

Both  the  composition  of  the  fluid  and  the  texture  of  the 
solid  parts  of  animal  and  vegetable  bodies  are  infinitely  va- 
ried, according  to  the  purposes  they  are  designed  to  serve  in 
the^conomy.    Scarcely  any  part  is  perfectly  homogeneous; 
that  is,  composed  throughout  of  a  single  uniform  material. 
Few  of  the  fluids  are  entirely  limpid,  and  none  are  perfectly 
simple  in  their  composition;  for  they  generally  contain  more 
or  less  of  a  gelatinous  matter,  which,  when  very  abundant, 
imparts  to  them  viscidity,  constituting  an  approach  to  the  so- 
lid state.    Many  fluids  contain  minute  masses  of  matter,  ge- 
nerally having  a  globular  shape,  which  can  be  seen  only  by 
means  of  the  microscope,  and  which  float  in  the  surround- 
ing liquid,  and  often  thicken  it  in  a  very  sensible  manner.! 
We  next  perceive  that  these  globules  have,  in  many  instances, 
cohered,  so  as  to  form  solid  masses;  or  have  united  in  lines, 
so  as  to  constitute  fibres.     We  find  these  fibres  collectins: 
and  adhering  together  in  bundles;  or  interwoven  and  agglu- 
tinated, composing  various  other  forms  of  texture;  sometimes 
resembling  a  loose  net-work  of  filaments;  sometimes  consti- 
tuting laminae  or  plates;  and,  at  other  times,  both  plates  and 
filaments   combining   to  form  an   irregular   spongy  fabric. 
These  various  tissues,  again,  may  themselves  be  regarded  as 
the  constituent  materials  of  which  the  several  organs  of  the 
body  are  constructed,  with  difierent  degrees  of  complication, 

*  See  a  paper  on  this  subject  by  Mr.  Bauer,  Phil.  Trans,  for  1823,  p.  1. 
f  Blauivillc,  Annates  des  Sciences  Naturelles;  X.  104. 
+  Globules  of  this  description  have  been  found  in  the  lymph,  the  saliva, 
and  even  in  the  aqueous  humour  of  the  eye. 


60  THE  MECHANICAL  FUNCTIONS. 

according  to  the  respective  functions  which  they  are  called 
upon  to  perform. 

We  shall  now  examine  the  several  kinds  of  texture  in  re- 
lation to  these  functions,  in  the  order  of  their  increasing 
complexity;  heginning  with  those  of  vegetables,  which  are 
apparently  the  simplest  of  all. 

§  2.    Vegetable  Organization, 

Plants,  being  limited  in  their  economy  to  the  functions 
of  nutrition  and  reproduction,  and  being  fixed  to  the  same 
spot,  and  therefore  in  a  comparatively  passive  condition,  re- 
quire for  the  performance  of  these  functions  mechanical  con- 
structions of  a  very  different  kind  from  those  which  are  ne- 
cessary to  the  sentient,  the  active,  and  the  locomotiv^ani- 
mal.  The  organs  that  are  essential  to  vegetables  are  those 
which  receive  and  elaborate  the  nutritive  fluids  they  require, 
those  which  are  subservient  to  reproduction,  and  also  those 
composing  the  general  frame-work,  which  must  be  super- 
added to  the  whole  for  the  purpose  of  giving  mechanical 
support  and  protection  to  these  finer  organizations.  As 
plants  are  destined  to  be  permanently  attached  to  the  soil, 
and  yet  require  the  action  both  of  air  and  of  light;  and,  as 
they  must  also  be  defended  from  the  injurious  action  of  the 
elements,  so  we  find  these  several  objects  provided  for  by 
three  descriptions  of  parts:  namely,  first,  the  Roots,  which 
fix  plants  in  their  situation;  secondly,  the  Stems,  which 
support  them  in  the  proper  position,  or  raise  them  to  the 
requisite  height  above  the  ground;  together  with  the 
branches  which  are  merely  subdivisions  of  the  stem;  and 
thirdly,  the  external  coverings,  which  correspond  in  their 
office  to  the  teguments,  or  skin  of  animals. 

The  simplest  and  apparently  the  most  elementary  texture 
met  with  in  vegetables  is  formed  of  exceedingly  minute  ve- 
sicles, the  coats  of  which  consist  of  transparent  membranes 
of  extreme  tenuity.  Fig.  3  is  a  highly  magnified  represen- 
tation of  the  simplest  form  of  these  vesicles.*    But  they  ge- 

*  These  cells  are  well  represented  in  the  engravings  which  illustrate  Mr. 


VEGETABLE  ORGANIZATION. 


61 


nerally  adhere  together  more  closely,  composing  by  their 
union  a  species  of  vegetable  cellular  tissue,  which  may  be 
regarded  as  the  basis  or  essential  component  material  of 
every  organ  in  the  plant.  This  cellular  structure  is  repre- 
sented in  figures  4  and  5,  as  it  appears  in  the  Fucus  vesicu- 
losus;  the  first  being  a  horizontal,  and  the  second  a  vertical 
section  of  that  plant.*  The  size  of  these  cells  differs  consi- 
derably in  different  instances.  Kieser  states  that  the  dia- 
meter of  each  individual  cell  varies  from  the  330th  to  the 
55th  part  of  an  inch;  so  that  from  3,000  to  100,000  cells 
would  be  contained  in  an  extent  of  surface  equal  to  a  square 
inch.  But  they  are  occasionally  met  with  of  different  sizes, 
from  even  the  1000th  part  of  an  inch  to  the  30th. 


In  their  original  state,  these  vesicles  have  an  oval  or  glo- 
bular form;  but  they  are  soon  transformed  into  other  shapes, 
either  by  the  mutual  compression  which  they  sustain  from 
being  crowded  into  a  limited  space,  or  from  unequal  expan- 
sions in  the  progress  of  their  development.  From  the  first 
of  these  causes  they  often  acquire  angles,  assuming  the 
forms  of  irregular  rhomboidal  dodecahedrons,  and  often  of 
hexagonal  prisms,  like  the  cells  of  a  honey-comb;  and  by 

Slack's  memoir  on  the  elementary  tissue  of  plants,  contained  in  the  49th  vo- 
lume of  the  Transactions  of  the  Society  of  Arts. 
*  De  Candollc,  Org-anographie  V^getale. 


62  THE  MECHANICAL  FUNCTIONS. 

the  second,  they  are  elongated  into  cylinders,  or  slowly  ta- 
pering cones,  thus  passing  by  insensible  gradations  into  the 
tubular  form.  Figures  6,  7,  and  8,  are  representations  of 
some  of  these  different  states  of  transition  from  the  one  to 
the  other.  These  various  modifications  of  the  same  elemen- 
tary texture  have  been  distinguished  into  several  classes  of 
cells,  and  dignified  by  separate  technical  denominations, 
which  I  shall  not  stop  to  specify,  as  it  does  not  appear  that 
they  have  as  yet  thrown  any  light  on  vegetable  physio- 
logy. 

Many  of  the  cells  are  fortified  by  the  addition  of  elastic 
threads,  generally  disposed  in  a  spiral  course,  and  adhering 
to  the  inner  surfaces  of  the  membranous  coats  of  the  cells, 
which  they  keep  in  an  expanded  state.  (See  Fig.  9.)  When 
the. membranes  are  torn,  the  fibres;  being  detached,  unrol 
themselves,  and  being  loosely  scattered  among  the  neigh- 
bouring cells,  give  the  appearance  of  fibrous  connexions 
among  these  cells,  which  did  not  originally  exist.  Simple 
membranous  cells,  containing  no  internal  threads,  are  often 
found  intermixed  with  these  fibrous  cells.  In  many  of  the 
cells,  again,  the  original  spiral  threads  appear  to  have  coa- 
lesced by  their  edges;  thus  presenting  a  more  uniform  sur- 
face, excepting  that  a  few  interstices  are  left,  where  the  pel- 
lucid membrane,  having  no  internal  lining,  presents  the  ap- 
pearance of  transverse  fissures  or  oval  perforations,  (Fig.  10.) 
Cells  of  this  description  are  said  to  be  reticulated  or  spot- 
ted, and,  together  with  those  having  more  regularly  formed 
spiral  threads,  are  very  abundantly  met  with  in  plants  be- 
longino;  to  the  tribe  of  Orchideae. 

It  has  been  much  disputed  whether  the  cells  of  the  vege- 
table texture  are  closed  on  all  sides,  or  whether  they  com- 
municate with  one  another.  Mirbel  has  given  us  delinea- 
tions of  what  appeared  to  him,  when  he  examined  the  coats 
of  the  cells  with  a  microscope,  to  be  pores  and  fissures.  But 
subsequent  observations  have  rendered  it  probable  that  these 
appearances  arise  merely  from  darker  portions  of  the  mem- 
branes, where  opaque  particles  have  been  deposited  in  their 
substance.     Fluids  gain  access  into  these  cells  by  transuding  , 


VEGETABLE  ORGANIZATION.  63 

through  the  membranes  which  form  their  sides,  and  not  by 
any  apertures  capable  of  being  detected  by  the  highest  pow- 
ers of  the  microscope. 

If  all  the  cells  consist  of  separate  vesicles,  as  the  con- 
curring observations  of  modern  botanists*  appear  to  have 
satisfactorily  established,  the  partitions  which  separate  them, 
however  thin  and  delicate,  must  consist  of  a  double  mem- 
brane, formed  by  the  adhesion  of  the  coats  of  the  two  con- 
tiguous vesicles.  But  as  these  coats  can  hardly  be  supposed 
to  adhere  in  every  point,  we  may  expect  to  find  that  spaces 
have  been  left  in  various  parts  between  them;  and  that  com- 
munications exist  to  a  certain  extent  between  all  these  spaces; 
so  as  to  compose  what  may  be  regarded  as  one  large  cavity. 
These  have  been  denominated  the  intercellular  spaces;  and 
they  have  been  supposed  to  perform,  as  will  hereafter  be 
seen,  an  important  part  in  the  functions  of  Nutrition. 

Fluids  of  different  kinds  occupy  both  the  cells  and  the 
intercellular  spaces.  The  contents  of  some  is  the  simple 
watery  sap;  that  of  others  consists  of  peculiar  liquids,  the 
products  of  vegetable  secretion:  and  very  frequently  they 
contain  merely  air.  In  many  of  the  cells  there  are  found 
small  opaque  and  detached  particles  of  the  substance  termed 
by  chemists,  Feciila,  of  which  starch  is  the  most  common 
example.  In  several  parts,  and  more  especially  in  the  leaves, 
and  in  the  petals  of  flowers,  the  material  which  gives  them 
their  peculiar  colour  is  contained  in  the  cells  in  the  form  of 
minute  globules.  De  Candolle  has  given  it  the  name  of 
Chromule.\ 

The  cells  of  the  ligneous  portion  of  trees  and  shrubs  are 
farther  incrusted  with  particles  of  a  more  dense  material, 
peculiar  to  vegetable  organization,  and  termed  Ligniiie,  It 
is  this  substance  which  principally  contributes  to  the  den- 
sity and  mechanical  strength  of  what  are  called  tlie  Woody 
Fibres,  which  consist  of  collections  of  fasiform,  or  tapering 
vessels,  hereafter  to  be  described,  surrounded  by  assemblages 

*  In  particular,  Treviramis,  Kioscr,  Link,  Du  I'etit  ThoiKU-s,  I'oUiiii,  Ainlci^ 
Dutrochet,  and  De  Candolle. 
f  Organographie,  Tom.  1,  p.  19. 


64  THE  MECHANICAL  FUNCTIONS. 

of  cells  thus  fortified,  and  the  whole  cohering  in  bundles,  so 
as  to  present  greater  resistance  to  forces  tending  to  displace 
them  in  the  longitudinal  direction  than  in  any  other. 

Most  of  the  plants  which  are  included  in  the  Linnean 
class  of  Cryptogamia  have  a  structure  exclusively  composed 
of  cells,  as  has  been  already  shown  in  the  Fucus  vesiculosus. 
But  the  greater  number  of  other  plants  have,  in  addition  to 
these  cells,  numerous  ducts  or  vessels,  consisting  of  mem- 
branous tubes  of  considerable  length,  interspersed  through- 
out every  part  of  the  system.  These  tubes  exhibit  different 
modifications  of  structure,  more  especially  with  regard  to 
the  form  of  the  fibres,  or  other  materials,  which  adhere 
to  the  inner  surface  of  their  membranes;  and  these  modifi- 
cations correspond  very  exactly  with  those  of  the  vesicles 
already  described  as  constituting  the  simpler  forms  of  vege- 
table tissue.  There  can  be  little  doubt,  indeed,  that  the  ves- 
sels of  plants  take  their  origin  from  vesicles,  which  become 
elongated  by  the  progress  of  development  in  one  particular 
direction;  and  it  is  easy  to  conceive  that  where  the  extre- 
mities of  these  elongated  cells  meet,  the  partitions  which  se- 
parate their  cavities  may  become  obliterated  at  the  points  of 
junction,  so  as  to  unite  them  into  one  continuous  tube  with 
an  uninterrupted  interior  passage.  This  view  of  the  forma- 
tion of  the  vessels  of  plants  is  confirmed  by  the  gradation 
that  may  be  traced  among  these  various  kinds  of  structures. 
Elongated  cells  are  often  met  with  applied  to  each  other 
endwise,  as  if  preparatory  to  their  coalescence  into  tubes. 
Sometimes  the  tapering  ends  of  fusiform  cells  are  joined 
kterally  (as  seen  in  Fig.  12,)  so  that  the  partitions  which  • 
divide  their  cavities  are  oblique.  At  other  times  their  ends 
are  broader,  and  admit  of  their  more  direct  application  to 
each  other  in  the  same  line,  being  separated  only  by  mem- 
branes passing  transversely;  in  which  case  they  present,  un- 
der the  microscope,  the  appearance  of  a  necklace  of  beads 
(Fig.  13.)  When,  by  the  destruction  of  these  partitions, 
their  cavities  become  continuous,  the  tubes  they  form  exhi- 
bit a  series  of  contractions  at  certain  intervals,  marking  their 
origin  from  separate  cells.     In  this  state  they  have  received 


VEGETABLE    ORGANIZATION. 


65 


the  names  of  moniliform,  Jointed  or  beaded  vessels.* 
Traces  of  the  membranous  partitions  sometimes  remain  where 
their  obliteration  has  been  only  partial,  leaving  transverse 
fibres.  The  conical  terminations  occasionally  observable 
in  the  vessels  of  plants  also  indicate  their  cellular  origin,  t 

14  15 


1 


The  membrane  constituting  the  tube  is  sometimes  simple, 
like  those  of  the  simple  cells:  but  it  frequently  contains 
fibres,  or  other  internal  coatings,  corresponding  to  those  met 
with  in  the  more  compound  cells.  The  vessels  in  which  the 
internal  fibres  run  in  a  spiral  direction  (Fig.  14,)  are  deno- 
minated trachese,  or  spiral  vessels;  or,  from  their  being  found 
very  constantly  to  contain  air,  they  are  often  called  air  tubes. 
Their  diameter  is  generally  between  the  1000th  and  the 
300th  part  of  an  inch.  These  spiral,  or  air  vessels,  pervade 
extensively  the  vegetable  system.  The  threads  they  con- 
tain are  frequently  double,  treble,  quadruple,  or  even  still 
more  numerous:  they  are  of  great  length,  and  when  the  ex- 
ternal membrane  of  the  vessel  is  divided,  they  may  easily 
be  drawn  out  and  uncoiled,  their  elasticity  enabling  them  to 
retain  their  spiral  shape.  The  object  of  this  structure  ap- 
pears to  be  that  of  keeping  the  cavity  of  the  tube  always 
pervious,  by  presenting  resistance  to  any  external  force  tend- 
ing to  compress  and  close  itij: 

*  Mirbel  g'avc  tlicm  the  name  of"  Vaisseaux  en  chapelet." 

f  This  theory  of  the  derivation  of  vessels  from  cells  was  first  advanced  by 
Treviranus. 

+  Vessels  are  sometimes  met  with  which  appear  to  be  formed  simply  by 
the  coils  of  a  spiral  fibre  in  close  juxtaposition,  and  unattached  to  any  ex- 
ternal envelope,  or  connecting"  membrane. 

Vol.  I.  9 


66  THE  MECHANICAL  FUNCTIONS. 

In  many  instances  the  inner  fibres  of  the  tube,  instead  of 
forming  a  continuous  spiral,  appear  in  the  shape  of  rings, 
succeeding  one  another  at  regular  intervals,  and  constituting 
what  are  called  annular  vessels  (Fig.  15.)  They  are  gene- 
rally larger  than  the  spiral  vessels.  In  other  cases,  as  was 
first  observed  by  Hedwig,  the  adjacent  coils  are  found  to  be 
closely  coherent  throughout  the  greatest  part  of  their  course; 
leaving,  however,  occasional  intervals,  where  the  external 
membrane,  being  unprotected,  appears,  from  its  transparency, 
as  if  spotted  or  perforated  in  various  places  (Fig.  16.)  Every 
intermediate  stage  may  occasionally  be  seen  in  the  transi- 
tion from  one  of  these  forms  to  the  other,  in  consequence  of 
the  various  kinds  of  convolution,  of  branchings,  or  of  trans- 
verse junctions  of  fibres,  as  well  as  the  greater  or  less  extent 
of  their  lateral  adhesions.  All  these  varieties  are  met  with, 
not  only  in  different  vessels,  but,  as  was  observed  by  Mol- 
denhavver  and  Kieser,  even  in  the  dififerent  portions  of 
the  same  vessel,  when  followed  by  the  eye  throughout  a 
great  extent  of  its  length.  Thus,  in  the  course  of  the  same 
tube,  (as  seen  in  Fig.  17,)  we  find  parts  exhibiting  spiral 
fibres,  which  in  other  parts,  bifurcate  and  again  unite;  and 
in  others,  again,  form  rings:  these  may  afterwards,  by  a 
closer  junction,  present  a  reticulated  appearance,  or  a  series 
of  transverse  lines,  which,  becoming  smaller  and  smaller, 
are  at  length  mere  points,  arranged  in  circular  rows  around 
the  cylindrical  surface  of  the  vessel.* 

What  are  called  the  woody  fibres  have  their  origin,  like  all 
other  parts  of  plants,  in  cells.  These  are  generally  fusiform, 
that  is,  of  the  shape  of  a  double  cone,  very  greatly  elongated, 
and  placed  close  and  parallel  to  one  another,  with  the  nar- 
row extremities  of  one  set  wedged  in  between  those  of  ano- 
ther set  (Fig.  18.)  Their  coats  are  more  firm  and  elastic 
than  those  of  ordinary  vessels,  but  do  not  appear  to  contain 
any  internal  fibres,  although  they  receive,  in  the  progress  of 

*  Many  disting-uisbed  botanists,  such  as  Rudolpbi,  Link,  Treviranus,  and 
Dutrocbet,  consider  these  spots  as  being  produced  not  by  the  deficiency  of 
the  internal  coating,  but  by  the  addition  of  granular  bodies.  See  De  Can- 
doUe's  Organographie  Vegetale,  torn.  1,  p.  56. 


VEGETABLE  ORGANIZATION.  67 

tlieir  development,  large  additions  of  solid  matter.     These 
fibres  are  generally  collected  together  into  bundles  or  layers, 
and  are  accompanied  by  cells  and  vessels  of  various  descrip- 
tions, and  in  different  stages  of  transition.     The  density  of 
the  woody  fibres  increases  in  proportion  as  these  incrusta- 
tions are  formed,  till  they  have  become  nearly  impervious; 
and  have  acquired  a  degree  of  rigidity  peculiarly  fitting  them 
for  the  office  of  giving  mechanical  support  to  the  fabric  of 
the  plant.*      Their  assemblage  thus  constitutes  a  kind  of 
frame-work  for  the  whole  system,  which  may  be  regarded 
as  the  skeleton  of  the  plant.    Thus,  what  are  called  the  fibres 
of  leaves  (Fig.  19,)  are  principally  composed  of  these  woody 
fibres,  distributed  in  the  manner  best  adapted  to  support  the 
expansion  of  the  soft  and  pulpy  substance  of  those  important 
organs. 

Besides  the  minute  cavities  of  the  cellular  tissue,  there 
occur,  in  various  parts  of  a  plant,  much  larger  spaces,  appa- 
rently serving  the  purpose  of  reservoirs  of  particular  fluids; 
but  sometimes  containing  only  air.  Large  air  cells  are^  in 
particular,  met  with  very  commonly  in  aquatic  plants,  where 
they  probably  contribute  to  impart  the  requisite  degree  of 
buoyancy. 

There  are  also  contained,  in  the  interior  of  vegetables, 
certain  organs,  denominated  Glands,  which  are  composed 
of  closely  compacted  cells,  and  which  perform  the  function 
of  secretion,  that  is,  the  conversion  of  the  nutritious  juices 
into  particular  products  required  for  various  purposes  in  the 
economy  of  the  plant. 

The  external  parts  of  a  living  plant  require  protection 
against  the  injurious  effects  of  the  atmosphere,  and  of  the 
moisture  it  deposites.  For  this  purpose  there  is  provided  a 
membrane,  termed  the  Cuticle,  which  is  spread  over  the 
whole  surface,  investing  the  leaves  and  flowers,  as  well  as 

*  By  dry'mg  diflTerent  specimens  of  wood  in  a  stove,  Count  Rumford  was  led 
to  the  conclusion  that  the  specific  gravity  of  the  solid  matter  which  consti- 
tutes timber  is  nearly  the  same  in  all  trees.  He  found  that  tlie  woody  part 
of  oak,  in  full  vegetation,  constitutes  only  two-fifths  of  the  whole  bulk:  and 
that  ordinary  dry  wood  contains  above  one-fourth  of  its  weight  of  water. 
Thomson's  Annals  of  Philosophy,  I.  388. 


68  THE  MECHANICAL  FUNCTIONS. 

the  stem  and  branches,  and  interpos'ing  a  barrier  to  the  ac- 
tion of  fluids,  or  other  extraneous  bodies,  on  the  living  or- 
gans. The  cuticle  is  formed  originally  by  the  condensation 
of  a  layer  of  cellular  tissue,  of  which  the  cells,  being  conso- 
lidated by  exposure  to  the  air,  and  by  compression,  compose 
a  thin  but  impervious  pellicle.  Amici  has  distinctly  shown, 
by  means  of  his  powerful  microscope,  the  cellular  structure 
of  the  cuticle,  and  also  that  the  layer  of  cells  of  which  it  con- 
sists is  independent  of  the  subjacent  cellular  tissue.*  Fig. 
20  is  intended  to  show  this  circumstance,  the  shaded  part 
representing  the  cuticle  with  its  series  of  cells. 

Oval  orifices,  or  stomata,  as  they  have  been  termed,  are 
discoverable  on  almost  every  part  of  the  surface  of  the  cuti- 
cle, but  more  especially  in  those  that  have  a  green  colour.! 
They  are  placed  at  nearly  equal  distances  from  one  another, 
and  are  particularly  numerous  in  the  cuticle  of  the  leaves, 
where  they  occupy  the  intervals  between  the  fibres.  These 
orifices  conduct  into  the  interior  of  the  plant,  probably  into 
the  general  cavity  of  the  intercellular  spaces.  It  is  evident, 
from  the  functions  they  perform,  that  they  must  occasional- 
ly open  and  close;  but  the  minuteness  of  their  size  precludes 
any  accurate  observation  as  to  the  nature  of  the  apparatus 
provided  for  the  purpose  of  performing  these  motions.  Ami- 
ci describes  their  margins  as  formed  by  two  cells,  by  the 
movements  of  which,  combined  perhaps  with  those  of  the 
adjoining  cells,  he  conceives  these  orifices  are  opened  and 
closed.!  Great  variety,  however,  is  observable  in  the  struc- 
ture of  the  stomata  in  different  species  of  plants. 

Many  plants  have  no  stomata,  either  on  the  cuticle  of  the 
leaves,  or  on  that  of  the  stem.  This  is  the  case  with  such 
aquatic  plants  as  are  habitually  immersed  in  water.  In  those 
that  are  only  partially  immersed,  stomata  are  met  with  in 
those  parts  exclusively  which  are  above  the  water.     The 

•  Annales  des  Sciences  Naturelles,  II.  211. 

•j-  Fig".  22  is  a  magnified  representation  of  the  appearance  in  the  cuticle 
of  the  Lycopodium  denticulatum^  tixken  in  the  central  part  of  the  lower  sur- 
face of  the  leaf,  from  De  Candolle.  Fig.  21  is  a  still  more  magnified  view 
of  the  stomata  in  the  leaf  of  the  Lilium  ca?ididum,  from  Amici. 

t  Ibid.  II.  215. 


VEGETABLE  orftANIZATION. 


69 


leaves  of  the  Ranunculus  aquaticus,  when  made  to  grow 
in  the  air,  acquire  stomata,  but  lose  them  entirely  when  grow- 
ing under  water.  Stomata  are  wanting  in  all  plants  whose 
structure  is  wholly  cellular. 


Botanists  are  far  from  being  agreed  as  to  the  precise  func- 
tions which  the  stomata  perform.  Their  usual  office  un- 
doubtedly is  to  exhale  water;  but  they  probably  also  absorb 
air  under  certain  circumstances,  and  in  particular  exigences. 

The  principal  organs  through  which  the  fluids  that  serve 
for  nourishment  are  received  into  the  system  of  plants,  are 
those  situated  at  the  extremities  of  the  roots,  where  they  are 
termed,  from  their  peculiar  texture,  spongioles.^  Of  the 
functions  of  spongioles  in  absorbing  fluids  I  shall  have  occa- 
sion to  speak  when  treating  of  nutrition.  But  as  the  roots 
exercise  a  mechanical  as  well  as  a  nutrient  office,  we  should 
here  consider  them  in  the  light  of  organs  adapted  to  procure 
to  the  plant  a  permanent  attachment  to  the  soil,  upon  which 
it  is  wholly  dependent  for  its  supply  of  nourishment.  It  is 
scarcely  necessary  to  point  out  how  effectually  they  perform 
this  office.  Our  admiration  cannot  fail  to  be  excited  when 
we  contemplate  the  manner  in  which  a  large  tree  is  chained 
to  the  earth  by  its  powerful  and  widely  spreading  roots.  By 
the  firm  hold  which  they  take  of  the  ground,  they  procure 

*  Fig.  23  exhibits  tlie  termination  of  a  root  of  a  willow  in  a  sponglole; 
tlie  arrangement  of  the  cells  composing  which  is  shown  in  Fig.  24,  from  Dc 
CandoUe. 


70  THE  MECHANICAL  FUNCTIONS. 

the  most  effectual  resistance  to  the  force  of  the  winds,  which, 
acting  upon  so  large  a  surface  as  that  presented  by  the 
branches  covered  with  dense  foliage,  must  possess  an  im- 
mense mechanical  power. 

The  principal  seat  of  the  vitality  of  a  plant  is  the  part 
which  intervenes  between  the  root  and  the  stem.  Injuries 
to  this  part  are  always  fatal  to  the  life  of  the  plant. 

As  the  roots  penetrate  downwards  into  the  earth  to  dif- 
ferent distances  in  order  to  procure  the  requisite  nourish- 
ment, so  the  stem  grows  upwards  for  the  purpose  of  obtain- 
ing for  the  leaves  and  flowers  an  ample  supply  of  air,  and 
the  influence  of  a  brighter  light,  both  of  which  are  of  the 
highest  importance  to  the  maintenance  of  vegetable  life. 
The  stems  of  the  grasses  are  hollow  tubes;  their  most  solid 
parts,  which  frequently  consist  of  a  thin  layer  of  silex,  oc- 
cupying the  surface  of  the  cylinder.  Of  all  the  possible 
modes  of  disposing  a  given  quantity  of  materials  in  the  con- 
struction of  a  column,  it  is  mathematically  demonstrable  that 
this  is  the  most  eff'ective  for  obtaining  the  greatest  possible 
degree  of  strength.* 

The  graceful  continuous  curve  with  which  the  stem  of  a 
tree  rises  from  the  ground,  is  the  form  which  is  best  calcu- 
lated to  give  stability  to  the  trunk.  Evidence  of  express 
mechanical  design  is  likewise  afforded  by  the  manner  in 
which  the  trunk  is  subdivided  into  its  branches,  spreading 
out  in  all  directions,  manifestly  with  a  view  to  procure  for 
the  leaves  the  greatest  extent  of  surface,  and  thus  enable  them 
to  receive  the  fullest  action  of  both  light  and  air.  The 
branches,  also,  are  so  constructed  as  to  yield  to  the  irregular 
impulse  of  the  wind,  and  again,  by  their  elasticity,  to  return 
to  their  natural  positions,  and  by  these  alternate  inflexions 
on  opposite  sides,  to  promote  the  motion  of  the  sap  in  the 
•vessels  and  cellular  texture  of  the  liber  and  alburnum.  No- 
thing can  exceed  the  elegance  of  those  forms  which  are  pre- 

*  Galileo,  the  most  profound  philosopher  of  his  age,  when  interrogated 
by  the  inquisition  as  to  his  belief  in  a  Supreme  Being-,  repUed,  pointing  to 
a  straw  on  the  floor  of  his  dungeon,  that  from  the  structure  of  that  object 
alone  he  would  infer  with  certainty  the  existence  of  an  intelligent  Creator. 


VEGETABLE  ORGANIZATION.  71 

sented  in  every  part  of  the  vegetable  kingdom,  whether  they 
be  considered  with  reference  to  their  direct  utility  for  the 
support  of  individual  life,  and  the  continuance  of  the  species, 
or  whether  they  be  viewed  as  component  parts  of  that  beau- 
ty which  is  spread  over  the  scenery  of  nature,  and  is  so  de- 
lightfully refreshing  to  the  eye  of  every  beholder  alive  to 
its  fascinating  charms.  How  enchanting  are  all  the  varieties 
of  flowers,  that  decorate  in  gay  profusion  every  part  of  the 
garden  of  creation;  and  into  which  the  farther  we  carry  our 
philosophic  scrutiny,  the  more  forcibly  will  our  hearts  be 
impressed  with  the  truth  of  the  divine  appeal  that  "  Even 
Solomon,  in  all  his  glory,  was  not  arrayed  like  one 

OF  THESE." 


§  3.  Development  of  Vegetables. 

Farther  proofs  of  design  may  be  collected  from  an  ex- 
amination into  the  modes  in  which  these  structures,  so 
admirably  adapted  to  their  objects,  have  been  gradually 
formed.  Confining  our  attention  to  vascular  plants,  in  which 
the  process  of  development  has  been  studied  with  the  great- 
est attention  and  success,  we  find  that  Nature  has  pursued 
two  different  plans  in  conducting  their  growth.*  In  the 
greater  number,  the  successive  additions  to  the  substance  of 
the  stem  are  made  on  the  exterior  side  of  the  parts  from 
^  which  they  proceed.  This  mode  is  adopted  in  what  are 
called  Exogenous ])lants.  In  others,  the  growth  is  the  re- 
sult of  additions  made  internally;  a  plan  which  is  followed 
in  all  Endogenous  plants.  The  Oak,  the  Elm,  the  Beech, 
the  Pine,  and  all  the  trees  of  these  northern  resiions,  belons 
to  the  first  of  these  divisions.  The  Palm  tribe,  such  as  the 
Date,  the  Cocoa-nut  tree,  and,  indeed,  a  large  proportion  of 
the  trees  of  tropical  climates,  together  with  the  sugar-cane, 
the  bamboo,  and  all  gramineous  and  liliaceous  plants,  belong 
to  the  latter.     We  shall  first  inquire  into  the  endogenous 

•  The  tribe  oii  Filices,  or  ferns,  the  structure  of  which  is  vascular,  consti- 
tutes an  exception  to  this  rule:  as  they  differ  in  their  mode  of  development, 
both  from  exogenous  and  endogenous  plants. 


72  THE  MECHANICAL  FUNCTIONS. 

mode  of  growth,  as  being  the  simplest  of  these  two  kinds  of 
vegetable  development. 

A  Palm  tree  may  be  taken  as  an  example  of  the  mode  of 
growth  in  endogenous  plants.  The  stem  of  this  tree  is  usu- 
ally perfectly  cylindrical,  attains  a  great  height,  and  bears  on 
its  summit  a  tuft  of  leaves.  It  is  composed  of  an  extremely 
dense  external  cylindric  layer  of  wood;  but  the  texture  of 
the  interior  becomes  gradually  softer  and  more  porous  as  it 
comes  nearer  to  the  centre;  though  wdth  regard  to  its  essen- 
tial character  it  appears  to  be  uniform  in  every  part,  having 
neither  medullary  rays,  nor  true  outward  bark,  nor  any  cen- 
tral pith;  in  all  which  respects  it  differs  totally  from  the  or- 
dinary exogenous  trees. 

The  first  stage  of  its  growth  consists  in  the  appearance  of 
a  circle  of  leaves,  which  shoot  upwards  from  the  neck  of 
the  plant,  and  attain,  during  the  first  year,  a  certain  size. 
The  following  year,  another  circle  of  leaves  arises;  but  they 
grow  from  the   interior  of  the   former  circle,  which  they 
force  outwards  as  their  vegetation  advances,  and  as  ligneous 
matter  is  deposited  within  them.     Thus,  each  succeeding 
year  brings  with  it  a  fresh  crop  of  leaves,  intermixed  with 
lio-neous  matter,  which  leaves,  exerting  an  outward  pressure, 
stretch  out  the  preceding  layers  that  enclose  them;  until  the 
latter  acquiring  greater  density,  no  longer  admit  of  farther  dis- 
tention, and  remain  permanently  fixed.  This  happens  first  to 
the  outermost  layer,  which  is  the  oldest:  then  each  succeeding  ^ 
layer  becomes  consolidated  in  its  turn.    As  soon  as  the  outer 
layer  has  become  too  hard  to  yield  to  the  pressure  from 
within,  the  growth  of  the  inner  layers  is  immediately  directed 
upwards;  so  that  they  each  rise  in  succession  by  distinct  stages, 
always  proceeding  from  the  interior;  a  mode  of  develop- 
ment which  has  been  compared  by  De  Candolle  to  the  draw- 
ing out  of  the  sliding  tubes  of  a  telescope.    The  whole  stem, 
whatever  height  it  may  attain,  never  increases  its  diameter 
after  its  outward  layer  has  been  consolidated.     A  circle  of 
leaves  annually  sprouts  from  the  margin  of  the  new  layer  of 
wood;  these,  when  they  fall  off  in  autumn,  leave  on  the  stem 
certain  traces  of  their  former  existence,  consisting  of  a  cir- 


DEVELOPMENT  OF  VEGETABLES.  73 

cular  impression  round  the  stem.  The  age  of  the  tree  may 
accordingly  be  estimated  by  the  number  of  these  circles,  or 
knots,  which  appear  along  its  stem.  The  successive  knots 
which  are  seen  in  the  stems  of  other  endogenous  plants,  as 
may  be  observed  in  growing  corn,  and  also  in  various  grasses, 
may  be  traced  to  a  similar  origin. 

The  structure  of  exogenous  trees  is  more  complicated: 
for,  when  fully  grown,  they  are  composed  of  two  principal 
parts,  the  toood  and  the  hark.  The  woody  portion  ex- 
hibits a  farther  division  into  the  j?;///?,  which  occupies  the 
centre,  and  consists  of  large  vesicles,  not  cohering  very 
closely,  but  forming  a  light  and  spongy  texture,  readily  per- 
meable to  liquids  and  to  air;  the  harder  luood,  which  sur- 
rounds the  pith  in  concentric  rings,  or  layers;  and  the 
softer  wood,  or  alhiirnum,  which  is  also  disposed  in  con- 
centric la)^ers  on  the  outer  side  of  the  former.  Each  of  these 
concentric  layers  of  wood  and  of  alburnum  may  be  farther 
distinguished  into  an  inner  and  an  outer  portion;  the  former 
being  of  less  density  than  the  latter,  and  consisting  of  a 
lighter  cellular  tissue:  while  the  outer  portion  is  composed  of 
the  denser  woody  fibres  resulting  from  the  union  of  numerous 
vessels  with  a  cellular  envelope.  The  bark  is  formed  b}^  con- 
centric layers  of  cortical  substance,  of  which  the  innermost 
are  denominated  the  Liber;  and  the  whole  is  surrounded 
by  an  outer  zone  of  cellular  tissue,  termed  the  cellular  en- 
velope. Of  this  envelope  the  exterior  surface  is  called  the 
Epidermis. 

All  these  concentric  zones  may  be  readily  distinguished  in 
a  horizontal  section  of  the  stem;  which  also  presents  a  num- 
ber of  lines  called  Medullary  Rays,  radiating  from  the  pith 
to  the  circumference.  They  are  composed  chiefly  of  large 
cells,  extending  transversely,  or  in  the  direction  of  the  di- 
ameter of  the  tree,  and  composing  by  their  union  conti- 
nuous vertical  planes  the  whole  length  of  the  trunk. 

Every  vegetable  stem,  and  also  every  branch  which  arises 
from  it,  is  developed  from  a  germ,  or  bud,  which  is  origi- 
nally of  inconceivable  minuteness,  and  totally  imperceptible 
by  any  optical  means  of  which  we  have  the  command.     As 

Vol.  I.  10 


74  THE  MECHANICAL  FUNCTIONS. 

soon  as  it  becomes  visible,  and  its  structure  can  be  distin- 
guished, it  is  found  to  contain  within  itself  the  parts  which 
are  to  arise  from  it,  in  miniature,  and  folded  up  in  the  small- 
est possible  compass.       The  portion  destined  to  form  the 
stem  is  gradually  expanded  both  in  breadth  and  height,  but 
principally  the  latter;   so  that  it  rises  as  it  grows,  during  a 
certain    period,  until    the    fibres    acquire  the    solidity  and 
strength  necessary  not  only  for  their  own  support,  but  also 
for  sustaining  the  parts  which  are  to  be  farther  added.     In 
trees  this   process  generally   occupies    one    whole    season; 
during  which  the  growth  of  the  first  layer  of  wood,  with  its 
central  pith,  and  its  covering  of  a  layer  of  bark,  is  free  and 
unrestrained.     On  the  second  year,  a  fresh  impulse  being 
given  to  vegetation,  a  new  growth  commences  from  the  up- 
per end  of  the  original  stem,  as  if  it  were  the  development 
of  a  new  bud:  and  at  the  sam^e  time  a  layer  of  cellular  tis- 
sue is  formed  by  the  deposition  of  new  materials  on  the 
outside  of  the  former  wood,  and  between  it  and  the  bark. 
This  is  followed  by  a  second  layer  of  wood,  enveloping  the 
new  layer  of  cellular  tissue. 

The  effect  of  this  new  growth  is  to  compress  the  layer  of 
wood  which  had  been  formed  during  the  first  year,  and  to 
impede  its  farther  extension  in  breadth.  But  as  its  fibres, 
consisting  of  vessels  and  cells,  are  not  yet  consolidated,  and 
admit  of  still  greater  expansion  as  long  as  they  are  supplied 
with  nourishment,  their  growth,  which  is  restrained  late- 
rally, is  now  directed  upwards,  and  there  is  no  farther  en- 
largement of  their  diameter.  From  the  same  cause  the  pith 
cannot  increase  in  size;  and  is  even  found  to  diminish  by 
the  pressure  of  the  surrounding  wood.  Thus,  the  vertical 
elongation  of  the  entire  stem  continues  during  the  whole  of 
the  second  year,  and  the  trunk  becomes  sufliciently  strength- 
ened by  the  addition  of  the  second  layer  on  its  outside  to 
bear  this  increase  of  its  height. 

While  this  process  is  going  on  in  the  w^ood,  correspond- 
ing changes  take  place  in  the  bark,  and  a  new  layer  is  add- 
ed on  its  inner  surface,  or  that  which  is  contiguous  to  the 
wood.  This  layer  constitutes  the  liber.  All  these  new  de- 
positions must  of  course  tend  to  stretch  the  outer  portions 


DEVELOPMENT  OF  VEGETABLES.  75 

of  the  bark,  which  had  been  first  formed,  and  which  yield 
to  this  pressure  to  a  certain  extent;  but,  becoming  tlicm- 
selves  consolidated  by  the  effects  of  the  same  pressure,  they 
acquire  increasing  rigidity;  and,  the  same  cause  continuing 
to  operate,  they  at  length  give  way,  in  various  places,  form- 
ing those  deep  cracks,  which  are  observable  in  the  bark 
of  old  trees,  and  which  give  so  rugged  an  appearance  to 
their  surface.  The  cuticle  has,  long  before  this,  peeled  off, 
and  has  been  succeeded  by  the  consolidated  layers  of  corti- 
cal envelope  which  form  the  epidermis.  But  the  epider- 
mis, which  is  continually  splitting  by  the  expansion  of  the 
parts  it  encloses,  itself  soon  decays,  and  is  constantly  suc- 
ceeded by  fresh  layers,  produced  by  the  same  process  of 
consolidation  in  the  subjacent  cortical  substance. 

During  the  third  and  each  succeeding  year,  the  same  pro- 
cess is  repeated;  new  layers  of  cellular  texture  and  of  woody 
fibres  are  deposited  around  those  of  the  preceding  year's 
growth,  and  a  new  internal  coating  is  given  to  the  liber  of 
the  bark.  The  compressing  power  continues  to  be  exerted 
on  the  internal  layers  of  wood,  directing  their  growth  ver- 
tically, while  they  are  capable  of  elongation,  and  can  be  sup- 
plied with  nourishment.  In  time,  however,  by  continued 
pressure,  and  accumulating  depositions  of  solid  matter,  the 
vessels  and  the  cells  become  less  and  less  pervious  to  fluids; 
till  at  length  all  farther  dilatation  is  prevented.  But  the  tree 
still  continues  to  enlarge  its  trunk  by  the  annual  accessions 
of  vigorous  and  expansible  alburnum,  and  to  take  its  station 
among  its  kindred  inhalntants  of  the  forest;  till,  arriving  at 
maturity,  its  majestic  form  towers  above  all  the  junior  or 
less  vigorous  trees.  ^' 

The  development  of  each  branch  takes  place  in  the  same 
manner,  and  by  the  same  kind  of  process,  as  that  of  the  trunk. 
The  buds  from  which  they  originate,  spring  from  .the  angle 

*  It  is  contended  by  Dr.  Uarwin  and  other  writers  on  vegetable  physiology'' 

that  each  annual  shoot  should  be  rcg-arded  as  a  collection  of  individual  buds, 

each  bud  being  a  distinct  individual  plant,  and  the  whole  tree  an  aggregation 

V  of  such  individuals.     I  shall  have  occasion  to  revert  to  this  question  when  I 

come  to  consider  the  subject  of  vegetable  nutrition. 


70  THE  MECHANICAL  FUNCTIONS. 

formed  by  the  stalk  which  supports  a  leaf,  and  which  is 
termed  by  botanists  the  axilla  of  that  leaf  A  law  of  sym- 
metry is  established  by  nature  in  the  development  of  all  the 
parts  of  plants.  The  leaves,  in  particular,  are  frequently 
observed  to  arise  in  a  circle,  or  symmetrically  round  the  pa- 
rent stem;  forming  what  is  termed  a  lohorl,  or,  in  botanical 
lano-uase,  a  verticillated  arrangement.  In  other  cases  the}^ 
are  found  to  have  their  origins  at  equal  intervals  of  a  spiral 
line,  which  may  be  conceived  to  be  drawn  along  the  stem, 
or  the  branch  from  which  they  grow.  When  these  inter- 
vals correspond  to  the  semi-circumference  of  the  stem,  the 
leaves  alternate  with  one  another  on  its  opposite  sides. 

The  stems  of  most  plants,  even  those  that  are  perfectly 
erect,  exhibit  a  tendency  to  a  spiral  growth.  This  is  obser- 
vable in  the  fibres  of  the  wood  of  the  pine,  however  straight 
may  be  the  direction  of  the  whole  trunk.  This  tendency  is 
shown  even  in  the  epidermis  of  the  cherry  tree,  for  it  may 
be  stripped  off  with  more  facility  in  a  spiral  direction  than 
in  any  other.  The  primitive  direction  of  the  leaves  of  en- 
dogenous plants  is  a  spiral  one.  It  is  particularly  marked 
also  in  the  stems  of  creepers  and  of  parasitic  plants,  which 
are  generally  twisted  throughout  their  whole  length;  a  dis- 
position evidently  conducive  to  the  purpose  of  their  forma- 
tion, namely,  that  of  laying  hold  of  the  objects  with  which 
they  come  in  contact,  and  of  twining  round  them  in  search 
both  of  nourishment  and  of  support.  The  twisted  stems  of 
the  hop  and  of  ivy  show  this  structure  in  a  remarkable 
degree,  and  the  purpose  for  which  this  tendency  was  given 
cannot  be  mistaken. 

A  conjecture  has  been  offered  that  this  tendency  to  a  spi- 
ral growth  might  be  the  effect  of  the  influence  of  the  sun's 
light  acting  successively  on  different  sides  of  the  plant,  in 
the  course  of  its  diurnal  motion.  In  these  northern  latitudes 
the  direction  of  that  motion  is  from  east  to  west;  or,  to  an 
observer  facing  the  south,  from  left  to  right.  That  light  has 
a  powerful  influence  in  determining  the  direction  of  the 
growth  of  all  the  parts  of  the  plant  which  are  above  ground, 
is  manifest  to  every  one  who  has  observed  the  habits  of  ve- 
getables.    If  a  growing  plant  be  placed  in  a  situation  where 


DEVELOPMENT  OF  VEGETABLES.  77 

the  light  reaches  it  only  on  one  side,  it  will  always,  by  de- 
grees, turn  itself  to  that  side,  as  if  eagerly  pressing  forward 
to  obtain  the  beneficial  action  of  that  agent.  The  leaves, 
whose  functions  in  a  more  especial  manner  require  its  ope- 
ration, will  always  be  found  turned  towards  the  light.  The 
branches  of  a  tree,  which  have  naturally  a  tendency  to  rise 
vertically,  have  this  tendency  modified  by  the  superior  at- 
traction of  the  light,  when  it  can  reach  them  only  laterally. 
Thus,  while  those  on  the  upper  part  spread  out  in  full  luxu- 
riance in  all  directions,  those  below  them  are  obliged  to  ex- 
pand more  in  a  lateral  direction:  and  this  is  still  more  the 
case  with  the  lowest  branches,  which  shoot  out  horizontal- 
ly to  a  considerable  distance  before  they  turn  upwards,  and 
present  their  leaves  to  the  light.  Often,  however,  from 
the  deficiency  of  this  necessary  agent,  their  growth  is  much 
stinted,  or  entirely  prevented.  The  operation  of  this  cause 
is  extensively  seen  in  the  interior  of  a  dense  forest. 

It  may  be  objected  to  the  theory  of  the  spiral  growth  be- 
ing the  result  of  the  sun's  motion,  that  were  it  so,  the  direc- 
tion of  the  spiral  would  always  be  the  same,  that  is,  ascend- 
ing from  left  to  right  with  reference  to  the  axis.  But  this 
is  not  found  to  be  the  case,  for  the  direction  of  the  turns, 
though  generally  constant  in  the  same  plant,  is  far  from  be- 
ing the  same  in  all.  Dr.  Wollaston  ingeniously  suggested 
that  a  verification  of  the  theory  would  be  obtained,  were  it 
found  that  plants  transported  from  tlie  southern  to  the  nor- 
thern hemispheres,  would  have  this  direction  reversed,-  for 
it  is  evident  that  the  motion  of  the  sun's  lisht  in  the  two 
hemispheres  is  in  opposite  directions;  being,  in  the  southern 
hemisphere,  from  right  to  left,  to  a  spectator  facing  the  me 
ridian  position  of  the  sun,  which  in  those  regions  is  to  the 
north.  But  the  facts  are  not  in  accordance  with  this  view 
of  the  subject;  so  that  we  may  consider  the  hypothesis  as 
untenable. 

The  roots  dificr  considerably  from  the  stems  both  in  their 
structure,  and  in  their  mode  of  growth.  They  exhibit,  in- 
deed, the  appearance  of  medullary  rays  and  of  concentric  lay- 
ers, but  they  are  destitute  of  any  central  pith,  and  they  have 
no  tracheae;  neither  does  their  surface  present  any  appear- 


78  THE  MECHANICAL  FUNCTIONS. 

ance  of  stomata.  They  increase  in  thickness  in  the  same 
way  as  the  stem  increases.  This  law  obtains  both  in  exo- 
genous and  endogenous  plants:  they  do  not,  however,  grow 
in  length  by  the  elongation  of  any  of  their  parts,  but  simply 
by  additions  made  to  their  extremities.  Their  ramifications 
are  not  the  result  of  the  development  of  buds,  as  are  the 
branches  of  the  stem;  but  they  arise  merely  from  the  addi- 
tional deposites  taking  different  directions.  Almost  every 
part  of  the  surface  of  the  stem  or  branches  may  shoot  forth 
roots  if  they  are  covered  with  earth,  and  properly  moistened, 
and  if  they  are  supplied  with  sap  from  the  circulating  system 
of  the  plant  itself.  It  is  observed,  however,  that  they  gene- 
rally grow  from  certain  points  on  the  surface  of  the  bark, 
which  appear  as  dark  spots,  and  are  termed  Lenticellss.^' 
Great  variety  exists  in  the  form,  and  disposition  of  roots  in 
different  families  of  plants,  according  to  the  particular  pur- 
poses they  are  intended  to  serve,  conform.ably  to  their  ge- 
neral functions  of  absorption  and  of  mechanical  support. 
Both  these  purposes  are  promoted  by  their  sending  out  from 
their  sides  numerous  fibrils,  or  lesser  roots,  which  increase 
their  firm  hold  upon  the  soil,  as  well  as  multiply  the  chan- 
nels for  the  introduction  of  nourishment. 

Nature  has  supplied  various  plants  with  certain  appen- 
dages to  the  above  mentioned  structures,  the  use  of  which 
are  for  the  most  part  sufficiently  obvious.  Of  this  descrip- 
tion are  the  tendrils,  which  assist  in  fixing  and  procuring 
support  to  the  stems  of  the  weaker  plants;  the  stipidde, 
which  protect  the  nascent  leaves;  and  the  hractese,  which 
perform  a  similar  office  to  the  blossom.  The  different  kinds 
of  hairs,  of  down,t  of  thorns,  and  prickles,  which  are  found 
on  the  surface  of  different  plants,  have  various  uses;  some 
of  which  are  easily  understood,  particularly  that  of  defend- 
ing the  plant  from  molestation  by  animals.  The  sting  of 
the  nettle  is  of  this  class;  and  its  structure  bears  a  striking 

*  This  name  was  g^iven  to  them  by  De  Candolle,  Annates  des  Sciences 
Naturelles,  VII.  1,  and  Organograpliie,  I.  94. 

f  The  finer  hairs,  and  filaments  of  down,  are  composed  of  elongated  cells, 
either  single,  or  several  conjplned  end  to  end. 


DEVELOPMENT  OF  VEGETABLES.  79 

analogy,  as  we  shall  afterwards  have  occasion  to  notice,  to 
that  of  the  poisonous  fangs  of  serpents. 

The  purposes  answered  by  the  down,  which  covers  a  great 
number  of  plants,  are  not  very  obvious.  It  perhaps  serves 
as  a  protection  from  the  injurious  effects  of  cold  winds  on 
the  tender  surface:  or  it  may  have  a  relation  to  the  deposi- 
tion of  moisture;  or,  as  it  may  be  farther  conjectured,  the 
number  of  points  which  are  thus  presented  to  the  air  may  be 
designed  to  convey  electricity  from  the  atmosphere,  or  to 
restore  the  electric  equilibrium,  which  may  have  been  dis- 
turbed by  the  processes  of  vegetation. 

In  the  smaller  parts  of  plants,  as  in  the  general  fabric  of 
the  whole,  we  find,  on  examination,  the  most  admirable  pro- 
vision made,  according  to  the  particular  circumstances  of  the 
case,  for  the  mechanical  objects  of  cohesion,  support,  and  de- 
fence. Thus,  the  substance,  of  the  leaf,  of  which  the  func- 
tions require  that  a  large  surface  be  expanded  to  the  air  and 
light,  is  spread  out  in  a  thin  layer  upon  a  frame-work  of 
fibres,  like  rays,  connected  by  a  net-work  of  smaller  fibrils, 
and  constituting  what  is  often  called  the  skeleton  of  the  leaf. 

In  all  these  vegetable  structures,  while  the  objects  appear 
to  be  the  same,  the  utmost  variety  is  displayed  in  the  means 
for  their  accomplishment,  in  obedience,  as  it  were,  to  the 
law  of  diversity  which,  as  has  been  already  observed,  seems 
to  be  a  leading  principle  in  all  the  productions  of  nature.  It 
is  more  probable,  however,  judging  from  that  portion  of  the 
works  of  creation  which  we  are  competent  to  understand, 
that  a  specific  design  has  regulated  each  existing  variation 
of  form,  although  that  design  may  in  general  be  utterly  be- 
yond the  limited  sphere  of  our  intelligence. 

§  4.  Animal  Organization. 

The  structures  adapted  to  the  purposes  of  vegetable  life, 
which  are  limited  to  nutrition  and  reproduction,  would  be 
quite  insufficient  for  the  exercise  of  the  more  active  func- 
tions and  higher  energies  of  animal  existence.  The  power 
of  locomotion,  with  which  animals  are  to  be  invested,  must 
alone  introduce  essential  differences  in  their  organization,  and 


so  THE  MECHANICAL  FUNCTIONS. 

must  require  a  union  of  strength  and  flexibility  in  the  parts 
intended  for  extensive  motion,  and  for  being  acted  upon  by 
poweiful  moving  forces. 

The  animal,  as  well  as  the  vegetable  fabric  is  necessarily 
composed  of  a  union  of  solid  and  fluid  parts.    Every  animal 
texture  appears  to  be  formed  from  matter  that  was  original- 
ly in  a  fluid  state:  the  particles  of  which  they  are  composed 
having  been  brought  together  and  afterwards  concreting  by 
a  process,  which  may,  by  a  metaphor  borrowed  from  physi- 
cal science,  be  termed  animal  crystallization.  Many  of  those 
animals,  indeed,  which  occupy  the  lowest  rank  in  the  se- 
ries, such  as  Medusae,  approach  nearly  to  the  fluid  state;  ap- 
pearing like  a  soft  and  transparent  jelly,  which  by  spontane- 
ous decomposition  after  death,  or  by  the  application  of  heat, 
is  resolved  almost  wholly  into  a  limpid  watery  fluid.*    More 
accurate  examination,  however,  w^ill  show  that  it  is  in  reali- 
ty not  homogeneous,  but  that  it  consists  of  a  large  propor- 
tion of  water,  retained  in  a  kind  of  spongy  texture,  the  indi- 
vidual fibres  of  which,  from  their  extreme  fineness  and 
uniformity  of  distribution,  can  with  difliculty  be  detected. 
Thus,  even  those  animal  fabrics  which  on  a  superficial  view 
appear  most  simple,  are  in  reality  formed  by  an  extremely 
artificial  and  complex  arrangement  of  parts.     The  progress 
of  development  is  continually  tending  to  solidify  the  struc- 
ture of  the  body.     In  this  respect  the  lower  orders  of  the 
animal  kingdom,  even  when  arrived  at  maturity,  resemble 
the  conditions  of  the  higher  classes  at  the  earliest  stages  of 
their  existence.     As  we  rise  in  the  scale  of  animals,  we  ap- 
proximate to  the  condition  of  the  more  advanced  states  of 
development  which  are  exhibited  in  the  highest  class. 

Great  efibrts  have  been  made  by  physiologists  to  discover 
the  particular  structure  which  might  be  considered  as  the 
simplest  element  of  all  the  animal  textures;  the  raw  mate- 
rial, as  it  were,  with  which  the  whole  fabric  is  wrought: 

*  Thus  a  Medusa,  weighing-  twenty  or  thirty  pounds,  will,  by  this  sort  of 
general  liquefaction,  be  found  reduced  to  only  a  few  grains  of  solid  matter. 
Peron,  Annales  du  Musee,  torn.  XV.  p.  43.  See  also  a  memoir  by  Quoy  and 
Gaimardf  Annales  des  Sciences  Naturelles,  torn.  I.  p.  245. 


ANIMAL  ORGANIZATION.  81 

but  their  labours  have  hitherto  been  fruitless.  Fanciful  hy- 
potheses in  abundance  might  be  adduced  on  this  favourite 
topic  of  speculation;  but  they  have  led  to  no  useful  or  satis- 
factory result.  Haller,  who  pursued  the  inquiry  with  great 
ardour,  came  to  the  conclusion  that  there  existed  what  he 
calls  the  simple  or  primordial  fibre,  which  he  represents  as 
bearing  to  anatomy  the  same  relation  that  a  line  does  to  ge- 
ometry. Chemical  analysis  alone  is  sufficient  to  overturn 
all  these  hypotheses  of  the  uniformity  of  the  proximate  ele- 
mentary materials  of  the  animal  organs:  for  they  are  found 
to  be  extremely  diversified  in  their  chemical  composition. 
Neither  has  the  microscope  enabled  us  to  resolve  the  pro- 
blem: for  although  it  has  been  alleged  by  manj^  observers 
that  the  ultimate  elements  of  every  animal  structure  con- 
sists of  minute  globules,  little  confidence  is  to  be  placed  in 
these  results  obtained  by  the  employment  of  high  magnify- 
ing powers,  which  are  open  to  so  many  sources  of  fallacy. 
That  globules  exist  in  great  numbers,  not  only  in  the  blood, 
but  in  all  animal  fluids,  there  can  be  no  doubt:  and  that 
these  globules,  by  cohering,  compose  many  of  the  solids,  is 
also  extremely 'probable.  But  it  is  very  doubtful  whether 
they  are  essential  to  the  composition  of  other  parts,  such  as 
the  fibres  of  the  muscles,  the  nerves,  the  ligaments,  the  ten- 
dons, and  the  cellular  texture:  for  the  most  recent,  and  ap- 
parently most  accurate  microscopical  observations  tend  to 
show  that  no  globular  structure  exists  in  any  of  these  tex- 
tures.* 

The  element  which  we  can  recognise  without  difficulty 
as  composing  the  greater  portion  of  animal  structures,  is  that 
which  is  known  by  the  name  of  the  cellular  texture.  Al- 
though bearing  the  same  designation  as  the  elementary  mate- 
rial of  the  vegetable  fabric,  it  differs  widely  from  it,  in  its 
structure  and  mechanical  properties.  It  is  not,  like  that  of 
plants,  composed  of  a  union  of  vesicles;  but  is  formed  of  a 
congeries  of  extremely  thin  laminae,  or  plates,  variously  con- 

*  See  the  Appendix  to  Dr.  Hodgkln  and  Dr.  Fisher's  translation  of  Ed- 
ward's work  on  the  Influence  of  Physical  Agents  on  Life,  p.  440. 

Vol.  I.  11 


S2  THE  MECHANICAL  FUNCTIONS. 

nected  together  by  fibres,  and  by  other  plates  which  cross 
25  them  in  different  directionSjleaving  cavities  or 

cells.  (Fig.  25.)    These  cells,  or  rather  inter- 
vening spaces,  communicate  freely  with  one 
another;   and,  in  fact,  may  be  considered  as 
one  common  cavity,  subdivided  by  an  infinite 
number  of  partitions   into  minute  compart- 
ments.   Hence  the  cellular  texture  is  through- 
out readily  permeable  to  fluids  of  all  kinds,  and  retains  these 
fluids  in  the  manner,  and  on  the  same  principle,  as  a  sponge. 
The  cellular  texture  is  not  only  the  element,  or  essential 
material   employed  by  nature  in  the  construction  of  all  the 
parts  of  the  animal  fabric;  but,  in  its  simplest  form,  it  con- 
stitutes the  general  medium  of  connexion  between  adjacent 
oro-ans,  and  also  between  the  several  parts  of  the  same  organ. 
Like  the  mortar  which  unites  the  stones  of  a  building,  the 
cellular  texture  is  the  universal  cement  employed  to  bind 
together  all   the  solid    structures.     Its    properties   are  ad- 
mirably adapted  to  the  mechanical  purposes  which  are  re- 
quired in  difierent  parts  of  the  frame:  and  these  properties 
are  variously  modified  and  adjusted  to   suit  the  particular 
exigencies  of  the  case.     When,  for  instance,  difierent  parts 
require  to  be  moveable  upon  each  other,  the  cellular  sub- 
stance interposed  between  them  has  its  state  of  condensation 
adapted  to  the  degree  of  motion  required.    That  which  con- 
nects the  muscles,  or  surrounds  the  joints,  and  all  other  parts 
concerned  in  extensive   action,  has  a  looser  texture,  being 
formed  of  broad  and  extensible  plates,  with  few  lateral  ad- 
hesions, and  leaving  large  interstices;  while  the  more  quies- 
cent organs,  the  plates  of  the  cellular  substance,  are  thin  and 
small,  the  fibres  short  and  slender,  and  their  intertexture 
closer  and  more  condensed. 

Besides  being  flexible  and  extensible,  the  cellular  texture 
is  also  highly  elastic,  a  property  which  is  exceedingly  advan- 
tageous in  the  construction  of  the  frame.  Not  only  the  dis- 
placement of  parts  is  resisted  by  their  elasticity,  but,  when 
displaced,  they  tend  to  return  to  their  natural  position.  This 
property  performs  a  more  important  part  in  the  mechanism 


ANIMAL  ORGANIZATION.  83 

of  the  animal  than  of  the  vegetable  system:  as  mlglit,  indeed, 
have  been  anticipated  from  the  more  active  and  energetic 
movements  required  by  the  functions  of  the  former. 

The  cellular  texture,  in  its  simple  form,  admits  of  the 
ready  transmission  of  fluids  through  it;  but  it  is  necessary, 
on  many  occasions,  to  interpose  a  barrier  to  their  passage. 
Such  barriers  are  provided  in  membranes^  which  are  merely 
modifications  of  the  same  material,  spread  out  into  a  con- 
tinuous sheet  of  a  closer  texture,  after  the  surfaces  of  the 
plates  have  been  brought  to  cohere  so  as  to  obliterate  all  the 
cellular  interstices,  and  become  impervious  to  fluids.  Though 
equally  flexible  and  elastic  with  the  original  texture  of 
which  it  is  formed,  the  membrane  has  acquired,  by  this  con- 
solidation, greater  strength  and  firmness,  properties  which 
adapt  it  to  a  great  number  of  important  purposes.* 

Membranes  are  extensively  employed  to  connect  distant 
organs,  and  often  serve  to  determine  the  direction   and  ex- 
tent of  their  relative  motions.     They  furnish  strong  cover- 
ings for  the  investment,  the   support,  and  the  protection  of 
all  the  important  organs  of  the   body.     What    Paley   has 
termed  ihe  package  of  the  organs  is  efiected  principally  by 
their  intervention.     Membranes  are  also  employed  to  line 
the  interior  of  all  the  large  cavities  of  the  body,  as  those  of 
the  chest,  and  of  the  abdomen,  or  lower  part  of  the  trunk 
containing  the  organs  of  digestion.     These  membranes,  af- 
ter lining  the  sides  of  their  respective  cavities,  are  reflected 
back  upon  the  organs  which  are  enclosed  in  those  cavities, 
so  as  to  furnish  them  with  an  external  coverino;.     Their  in- 
ner  sides  present  every  where  a  smooth  and  polished  sur- 
face, over  which   the  organs   contained   in   the   cavity  may 
glide  without  injury.     In  all  these  cases,  a  thin  fluid,  called 
serum,  is  provided,  which  moistens  and  lubricates  the  sur- 
faces that  are  in  contact  with  one  another,  and  obviates  the 
injury  that  would  otherwise  arise  from  friction.     From  this 

*  With  a  view  of  ascertaining-  the  actual  strength  of  membranes,  Scarpa 
stretched  a  portion  of  peritoneum,  (which  is  a  very  thin  membrane  lining 
the  abdominal  cavity,)  over  a  hoop,  and  placing-  weights  upon  its  surface, 
found  it  did  not  give  way  till  it  was  loaded  with  fifteen  pounds. 


84 


THE  MECHANICAL  FUNCTIONS. 


circumstance,  the  linings  of  these  cavities  have  been  termed 
serous  mevibranes.  In  the  neighbourhood  of  joints,  closed 
cavities  of  the  same  description,  but  of  smaller  size,  are  met 
v^^ith,  for  the  obvious  purpose  of  facilitating  motion;  and 
here  also  friction  is  prevented  by  a  highly  lubricating  fluid, 
termed  synovia,  which  is  poured  out  between  the  surfaces 
of  the  membrane  lining  the  cavities. 

Membranes  being  impermeable  to  fluids,  are  extensively 
employed  as  receptacles  for  retaining  them:  forming,  in  the 
first  place,  sacs,  or  pouches  of  various  kinds  for  that  pur- 
pose. The  ink-bag  of  the  cuttle  fish,  the  gall-bladder,  and 
even  the  stomach  itself,  are  examples  of  this  kind  of  struc- 
ture. The  coats  of  these  sacs,  being  very  extensible  and 
elastic,  readily  accommodate  themselves  to  the  variable  bulk 
of  their  contents. 

In  the  second  place,  we  find  membranes  composing  tubes 
of  various  descriptions  for  conducting  fluids.  Thus,  in  the 
higher  classes  of  animals,  the  whole  of  the  body  is  traversed 
by  innumerable  canals  conveying  different  kinds  of  fluids. 
These  canals,  when  uniting  into  trunks,  or  subdividing  into 
branches,  are  called  Vessels,  (Fig  26.) 


The  fluids  contained  in  vessels  are  never  stagnant,  but  are 
almost  always  carried  forwards  in  one  constant  direction. 
For  preventing  the  retrograde  motions  of  the  fluids  passing 
along  these  canals,  recourse  is  had  to  the  beautiful  con- 
trivance of  valves.  The  inner  membrane  of  the  vessel  is 
employed  to  construct  these  valves;  for  which  purpose  it  is 
extended  into  a  fold  having  the  shape  of  a  crescent;  fixed  by 
its  convex  edge  to  the  sides  of  the  vessel,  while  the  other 
edge  floats  loosely  in  its  cavity.     Whenever  the  fluid  is  im- 


ANIMAL  ORGANIZATION.  (  85 

pelled  in  a  direction  contrary  to  its  proper  course,  it  raises 
the  loose  edge  of  the  valve,  which,  being  applied  to  the  op- 
posite side  of  the  canal,  effectually  closes  the  passage.  On 
the  contrary,  it  presents  no  obstacle  to  the  natural  flow  of 
the  contents  of  the  vessel,  both  edges  being  then  closely  ap- 
plied to  the  same  side.  Frequently  two,  or  even  three 
valves  are  used  at  the  same  part,  their  edges  being  made  to 
meet  in  the  middle  of  the  passage,  like  the  flood-gates,  or 
locks  of  a  canal.*  Among  the  numberless  instances  of  ex- 
press contrivance  which  are  met  with  in  the  examination  of 
the  fabric  of  animals,  there  is  perhaps  none  more  striking 
and  more  palpable,  than  this  admirable  mechanism  of  the 
valves. 

As  we  ascend  from  the  simpler  to  the  more  complicated 
systems  of  organization,  adapted  to  a  greater  range  of  facul- 
ties, we  find  greater  diversity  in  the  mechanical  means  em- 
ployed for  carrying  on  the  functions  of  life.  Textures  of 
greater  strength  than  can  be  constructed  by  membranes 
alone  become  necessary  for  the  security,  the  support,  and 
the  defence  of  important  organs;  and  more  especially  for  the 
execution  of  extensive  movements.  For  obtaining:  these 
advantages  a  peculiar  species  of  fibres  is  provided,  formed 
of  a  much  denser  substance  than  even  the  most  consolidated 
forms  of  cellular  texture.  The  animal  product  termed  albu- 
men possesses  a  much  stronger  cohesive  power  than  gelatin^ 
which  is  the  basis  of  membrane.  The  addition  of  albumen, 
therefore,  procures  the  quality  required:  and  the  fibres  that 
are  produced  by  its  combination  with  gelatin  are  opaque, 
and  of  a  glistening  white  colour.  By  interlacing  fibres  thus 
composed,  a  close  texture  is  formed,  which  is  exceedingly 
tough  and  unyielding.  Th^sQ  Jibrous  textures,  as  they  are 
termed,  while  they  retain  the  flexibility  of  membranes, 
greatly  surpass  them  in  strength;  but,  being  at  the  same  time 
incapable  of  extension,  they  are  necessarily  devoid  of  elasti- 

*  Fig.  27,  representing"  the  section  of  a  vessel,  is  Intended  to  show  the  po- 
sition of  the  valves  when  applied  lo  the  sides  of  the  vessel,  by  the  stream 
nnoving-  onwards  in  the  direction  pointed  out  by  the  arrow.  In  Fig".  28,  they 
are  seen  closing  the  passage  by  the  retrograde  pressure  of  tlie  current. 


86  THE  MECHANICAL  FUNCTIONS. 

city.  Hence,  they  are  adapted  to  form  external  tunics  for 
the  investment  of  such  organs  as  are  not  intended  to  vary  in 
their  size.  Occasionally,  these  fibrous  capsules,  as  they  are 
called,  send  down  processes  into  the  interior  of  those  organs, 
for  the  purpose  of  giving  them  mechanical  support.  This 
is  the  case,  for  instance,  with  the  membranes  surrounding 
the  brain  of  quadrupeds,  and  which  form  two  partitions,  the 
one  vertical,  the  other  horizontal;  both  being  firmly  stretched 
in  their  respective  positions,  and  serving  to  divide  the  pres- 
sure. In  other  cases  these  sheets  of  fibrous  membrane  are 
employed  as  bandages,  tightly  bracing  the  muscles,  and  re- 
taining them  in  their  relative  situations.  The  joints  are  sur- 
rounded by  similar  bandages,  known  by  the  name  of  Cap- 
sular Ligaments. 

In  following  the  series  of  animal  structures  in  the  order 
of  their  increasing  density,  we  find  the  proportion  of  albu- 
men which  enters  into  their  composition  becoming  greater, 
while  that  of  the  gelatin  and  mucilage  diminishes.  When 
the  product  is  more  uniform  in  its  composition  it  is  in  gene- 
ral less  elastic  than  when  it  consists  of  a  more  complex  com- 
bination of  ingredients.  A  great  preponderance  of  albumen 
tends  also  to  diminish  the  elasticity.  Thus,  the  densest 
kinds  of  fibrous  texture  present,  instead  of  thin  and  broad 
expansions  of  elastic  membrane,  the  thick  and  elongated 
form  of  inextensible  cords,  constituting  the  ordinary  Liga- 
vients,  and  the  Tendons.  These  structures  resist  with 
great  power  any  force  calculated  to  extend  them:  a  proper- 
ty which  of  course  excludes  elasticity,  but,  when  united 
with  flexibility,  implies  great  toughness.  In  a  word,  they 
possess  all  the  qualities  that  can  be  desired  in  a  rope.  It 
will  hardly  be  credited  how  great  a  force  is  required  to 
stretch,  or  rather  rend  asunder  a  ligament;  for  it  will  not 
yield  in  any  sensible  degree  until  the  force  is  increased  so 
enormously  as  at  once  to  dissever  the  whole  contexture  of 
its  fibres.  Nothing  can  be  more  artificially  contrived  than 
the  interweaving  of  the  fibres  of  ligaments;  for  they  are  not 
only  disposed,  as  in  a  rope,  in  bundles  placed  side  by  side, 
and  apparently  parallel  to  each  other:  but,  on  careful  exami- 


ANIMAL  ORGANIZATION.  87 

nation,  they  are  found  to  be  tied  together  by  oblique  fibres 
curiously  interlaced,  in  a  way  that  no  art  can  imitate.  It  is 
only  after  long  maceration  in  water,  that  this  complicated 
and  beautiful  structure  can  be  unravelled. 

The  mechanical  properties  of  these  fibrous  structures, 
which  are  strictly  inextensible  ligatures,  render  them  appli- 
cable to  purposes  of  connexion  where  motion  is  to  be  re- 
strained. Many  cases,  however,  occur  in  which  a  substance, 
is  wanted,  uniting  great  compactness  and  strength  with  a 
considerable  degree  of  elastic  power.  For  this  purpose  a 
different  texture  is  fabricated,  consisting  of  twisted  fibres, 
which  impart  this  required  elasticity.  Such  is  the  structure 
of  the  elastic  ligaments  of  animals,  which  are  very  gene- 
rally employed  for  the  support  of  heavy  parts  that  require 
being  suspended.  An  instance  occurs  in  quadrupeds,  in  that 
strong  ligament  which  passes  along  the  back  and  neck  to  be 
fixed  to  the  head,  and  to  support  its  weight  when  the  ani- 
mal stoops  to  graze.  This,  the  ligamentum  nitchde,  as  it  is 
termed,  is  capable  of  great  extension,  and  by  its  elasticity 
reacts  with  considerable  force  in  recovering  its  natural  length, 
after  it  has  been  stretched.  This  ligament  is  particularly 
strong  in  the  Camel,  whose  neck  is  of  great  length.*  Ano- 
ther example  of  an  elastic  ligament  occurs  in  that  which  con- 
nects the  two  shells  of  bivalve  mollusca  (as  those  of  the  oy- 
ster and  muscle,)  and  which  keeps  them  open  when  the  ani- 
mal exerts  no  force  to  close  them.  The  claws  of  the  Lion, 
and  other  animals  of  the  cat  tribe,  are  retracted  within  their 
sheaths  by  means  of  two  strong  elastic  ligaments.  Structures 

*  Many  birds  are  provided  with  strong-  elastic  lig-aments  connecting-  the 
vertebrae  of  the  neck  with  those  of  the  back;  ligaments  of  the  same  kind  are 
also  employed  for  retaining  the  wing-s  close  to  the  body,  where  they  are  not 
used  in  flying-:  and  a  similar  provision  is  made  in  the  wing-s  of  bats.  The 
weight  of  the  bulky  org-ans  of  digestion  in  herbivorous  quadrupeds  require 
some  permanent  support  of  this  kind;  and  this  is  furnished  by  a  broad,  elas- 
tic fibrous  band  extended  across  the  lower  part  of  the  abdomen.  It  is  par- 
ticularly strong  in  the  elephant,  which  remains  more  constantly  in  the  liori- 
zontal  position  than  most  quadrupeds:  and  it  has  been  remarked  that  the  g-e- 
neral  cellular  texture  in  this  animal  has  an  unusual  degree  of  elasticity. — 
Hunter  on  the  Blood,  &c.  p.  112. 


S8  THE  MECHANICAL  FUNCTIONS. 

of  this  kind  are  employed  very  extensively  in  the  fabric  of 
insects.* 

The  animal  substance  which  comes  next  in  the  order  of 
density  is  Cartilage.  The  purposes  for  which  this  kind  of 
structure  is  employed  are  those  in  which  a  solid  basis  is  re- 
quired for  the  support  of  softer  or  more  flexible  parts,  and 
where  the  mechanical  properties  that  are  wanted  are  firmness, 
conjoined  with  some  degree  of  elasticity.  Cartilage  (or  gris- 
tle) is  composed  of  a  finer  and  more  uniform  material  than 
any  of  the  preceding  structures.  It  consists  almost  wholly 
of  albumen,  with  a  slight  proportion  of  calcareous  matter. 
Unlike  membrane  in  any  of  its  forms,  it  contains  no  fibres, 
but,  on  being  cut  with  a  sharp  knife,  presents  the  appearances 
of  a  dense  homogeneous  substance  of  a  white  pearly  hue. 
Its  surface  is  smooth,  and  where  it  is  exposed  to  friction,  as 
in  the  joints,  is  often  highly  polished. 

In  all  the  inferior  tribes  of  animals  Nature  employs  car- 
tilage to  supply  the  place  of  bone  when  ridigity  is  required 
to  be  given  to  the  fabric.  In  an  extensive  order  of  fishes, 
including  the  shark,  the  sturgeon,  and  the  ray,  we  find  the 
whole  skeleton  constructed  of  cartilage.  In  the  fabric  of 
very  young  quadrupeds  cartilage  is  substituted  for  bone;  and 
in  the  adult  animal,  various  organs,  such  as  the  external  ears, 
the  eye-lids,  the  nostrils,  and  difierent  parts  of  the  apparatus 
of  the  throat  and  windpipe,  are  composed  of  flexible  carti- 
lage, which  gives  them  a  determinate  shape  and  firmness.  In 
all  these  cases  bone,  which  besides  being  three  times  as  hea- 
vy, is  devoid  of  elasticity,  and  liable  to  fracture,  w^ould  have 
been  much  less  suitable.  Cartilage  is  often  employed  as  an 
intermedium  for  connecting  difierent  bones,  as  for  instance, 
between  the  ribs  and  the  sternum,  or  breast  bone;  whereby, 
besides  the  advantage  of  greater  lightness,  the  pliancy  of 
the  material  diminishes  those  jars  which  are  incident  to  the 
frame  in  all  its  violent  actions. 

In  the  construction  of  cartilage,  nature  seems  to  have  at- 
tained the  utmost  degree  of  density  which  could  be  given 
to  an  internal  texture  composed  merely  of  the  usual  animal 

*  Chabrier,  Memolres  du  Musee,  torn.  vi.  p.  416. 


ANIMAL  ORGANIZATION.  89 

constituents.    But  substances  of  still  greater  hardness,  united 
with  perfect  rigidity,  are  wanted,  in  numberless  instances 
for  giving  effectual  protection  to  soft  and  delicate  structures, 
for  supplying  a  firm  basis  to  the  framework  of  the  body,  and 
for  constructing  levers  of  various  kinds  to  be  employed  in 
the  more  energetic  movements  of  the  higher  animals.     For 
all  these  purposes  it  was  necessary  to  superadd  a  material 
endowed  with  stronger  cohesive  powers,  and  capable  by  its 
dense  concretion  of  forming  solid   and   Inflexible   organs. 
The  substances  which  nature  has  selected  for  this  office  are 
the  salts  of  lime.    Sometimes  the  Carbonate,  and  sometimes 
the  Phosphate  of  lime  is  employed  for  forming  these  hard 
and  unyielding  structures;   and  often  both  these  calcareous 
substances  are  united  together  in  different  proportions  in  the 
same  solid  fabric.     When  the  carbonate  of  lime  predomi- 
nates, or  is  the  sole  earthy  ingredient,  it  constitutes  Shell: 
when  there  is  a  greater  proportion  of  the  phosphate,  it  is 
called  a  Crust,  as  is  the  case  with  the  coverings  of  the  lob- 
ster and  the  crab:  when  the  earthy  matter  consists  almost 
wholly  of  phosphate  of  lime,  it  composes  the  different  forms 
oiBone.     I  shall  have  occasion  to  describe  the  formation 
and  properties  of  each  of  these  structures  in  the  sequel. 

The  protection  of  the  delicate  structure  of  the  fabric  from 
the  injurious  influence  of  external  agents  is  an  object  of 
great  imjDortance  in  the  animal  economy,  and  is  one  which 
nature  has  shown  extreme  solicitude  to  secure.  For  this 
purpose  she  has  provided  the  integuments,  under  which 
designation  are  included  not  merely  the  skin,  but  also  all  the 
parts  that  are  immediately  connected  with  it,  and  are  formed 
and  nourished  by  the  same  vessels.  No  parts  of  the  animal 
structure  present  greater  diversity  in  their  form  and  out- 
ward appearance  than  the  integuments;  yet  it  is  easy  to  dis- 
cover, amidst  all  these  varieties,  that  the  same  general  plan 
has  been  followed  in  their  construction,  and  that  each  par- 
ticular formation  is  the  result  of  a  combination  of  the  same 
elementary  structures.  Of  these  elements  the  most  important, 
and  that  which  generally  composes  the  chief  bulk  of  the 
skin,  is  the  Corium,  or  true  skin.  The  outermost  layer  is 
Vol.  I.  12 


90  THE  MECHANICAL  FUNCTIONS. 

termed  the  Epidermis,  Cuticle,  or  scarf-skin;  and  between 
these  there  is  often  found  an  intermediate  layer  denominated 
the  Rete  Mucosum,  or  the  Pigmentum. 

The  corium  is  generally  of  considerable  thickness,  and  is 
composed  of  strong  and  tough  fibres,  closely  compacted  to- 
gether, and  pervaded  by  innumerable  ramifications  of  blood- 
vessels of  every  kind.    It  is  endowed  with  great  flexibility, 
and  is  capable  of  being  considerably  extended;  properties 
which  fit  it  for  readily  accommodating  itself  to  all  the  move- 
ments of  the  body  and  limbs,  and  to  the  variable  bulk  of  the 
parts  it  covers.     Being  also  very  elastic,  it  soon  regains   its 
natural  form  and  dimensions,  when  left  to  itself  after  being 
stretched.    The  skin  is  connected  with  the  subjacent  muscles 
and  other  parts  by  a  large  quantity  of  cellular  texture,  which, 
according  to  the  particular  intentions  of  its  formation,  some- 
times binds  it  tightly  over  these  parts,  and  on  other  occa- 
sions allows  of  a  free  and  extensive  motion.     This  latter 
property  is  remarkably  exemplified  in  the  Racoon,  an  ani- 
mal in  wdiich  the  skin  hangs  loosely  on  the  limbs,  and  en- 
closes the  body  like  a  wide  elastic  garment;  so  that,  however 
firmly  a  person  may  attempt  to  grasp  the  animal  by  the  neck, 
it  can  easily  turn  its  head  completely  round,  and  bite  the 
fino-ers  that  are  holding  it.     In  like  manner  the  skin  of  the 
frog  is  attached  to  the  body  only  at  a  few  places,  and  may 
be  readily  stripped  off".     A  thin  layer  of  muscular  fibres 
is  often  found  lying  immediately  underneath  the  skin,  and 
is  provided  for  the  purpose  of  moving  it  over  the  subja- 
cent parts.     In  animals  that  roll  themselves  into  a  ball,  as 
the  hedge-hog,  these  muscles  are  of  great  size  and  import- 
ance.    We  shall  see  that  in  the  mollusca,  this  muscular  ap- 
paratus  is  inseparably  blended  with  the  integument,  and 
composes  a  peculiar  structure,  termed  the  mantle.     Imme- 
mediately  covering  the  corium  is  the  Rete  Mucosum,  which 
is  a  very  thin  layer  of  soft  animal  matter,  composed  of  a  net- 
work of  delicate  fibres,  and  containing  more  or  less  of  the 
material  from  which  the  colour  of  the  skin  is  derived. 

The  Epidermis  is  a  membrane  of  a  very  peculiar  nature, 
consisting  of  a  thin  expansion  of  albuminous  matter  appa- 


ANIMAL  ORGANIZATION-.  [)l 

rently  homogeneous  in  its  texture  and  composition.  It  is 
impervious  to  fluids,  althougli  capable  of  imbibinsr  moisture, 
and  of  slowly  transmitting  a  portion  to  the  subjacent  tex- 
tures. Its  thickness  varies  exceedingly  in  different  parts; 
being  adapted  to  the  kind  of  protection  it  has  to  afford 
against  pressure,  friction  or  other  causes  of  injury.  As  it 
is  not  nourished  by  vessels,  its  outer  layer  is  liable  to  wear 
away,  or  to  become,  by  drying,  unfit  for  use:  and  accord- 
ingly a  separation  of  this  outward  layer  generally  takes  place 
from  time  to  time,  the  loss  being  speedily  repaired  by  a 
fresh  growth  from  the  surface  in  contact  with  the  skin. 
This  process  is  often  performed  periodically,  as  is  most  re- 
markably exemplified  in  serpents. 

Special  provisions  are  made  for  preserving  the  cuticle  in 
a  healthy  condition;  and  more  particularly  for  defending  it 
from  the  injurious  action  of  the  surrounding  element.    These 
sometimes  consist  of  a  supply  of  oily  fluid,  prepared   in 
small  cavities  that  are  situated  in  the  skin  itself,  and  have 
minute    ducts    opening  upon  the  surface.     These  cavities, 
termed    sebaceous  follicles,  are  generally    interspersed  in 
great  numbers  on   different  parts  of  the   body,  abounding 
more  especially  in  those  places  where  folds  occur,  and  where 
there  is  the  greatest  friction.     In  fishes,  mollusca,  and  other 
aquatic  animals,  the  skin  is  at  all  times  defended  from  the 
action  of  the  water,  by  a  viscid  or  glutinous  secretion   pre- 
pared in  this  m.anner,  and  continually  poured  out  on  the  sur- 
face, through  ducts,  the  orifices  of  which  arc  easily  seen  with 
the  naked  eye,  disposed  in  a  line  on  each  side  of  the  body. 
Connected  v/ith  the  skin,  and  more  particularly  with  the 
cuticle,  are  structures   of  very  various  forms,  intended  for 
giving  additional  protection,  occasionally  contributing  their 
aid  in  progressive    motion,  and  sometimes  fashioned  into 
weapons  of  offence.     In  this  class  should  bo  included  all  the 
varieties  of  hair,  such   as  wool,  fur,  feathers,  bristles,  quills, 
and  spines,  as  well  as  the  more  ordinary  kinds  of  hair.    All 
these  resemble  the  cuticle  in  their  chemical  composition, 
differing  only  in  their  degrees  of  hardness  and  condensation. 
Horn  is  formed  of  the  same  material  as  hair:  as  arc  also  the 


92 


THE  MECPIANICAL  FUNCTIONS, 


nails,  the  hoofs,  and  the  claws  of  quadrupeds,  and  the  scales 
of  fishes,  reptiles,  and  other  animals.  The  integuments  of 
insects,  and  especially  their  more  solid  and  horny  coverings, 
contain,  however,  as  will  hereafter  be  noticed,  a  peculiar 
chemical  principle  termed  Entornoline. 

All  these  parts  seem  to  be  but  remotely  connected  with 
the  vital  actions  of  the  system  with  which  they  are  associ- 
ated; and  it  is  doubtful  how  far  they  are  to  be  considered  as 
appertaining  to  the  living  portion  of  the  body,  or  as  mere 
extraneous  appendages.  Yet,  however  they  may  difier  in 
their  forms,  uses,  and  external  appearance,  they  all  are  pro- 
duced by  the  same  kind  of  vascular  structure,  variously  ar- 
ranged to  suit  the  particular  circumstances  in  each  case:  and 
the  mode  of  their  development  and  growth  is  essentially  the 
same  in  all. 

An  extremely  delicate  and  finely  organized  pulp,  com- 
posed partly  of  a  congeries  of  minute  vessels,  and  partly  of 
a  gelatinous  substance,  in  which  these  vessels  are  embedded, 
constitutes  the  apparatus  by  which  the  nutrient  particles  are 
selected,  combined  and  elaborated  into  the  materials  of  the 
intended  structure.  The  original  form,  situation,  and  dis- 
position of  this  vascular  pulp,  determines  the  future  figure 
and  extent  of  growth  of  the  production  which  is  to  arise 
from  it.  The  materials  w^iich  compose  it  are  deposited 
sometimes  in  masses,  as  in  the  scales  of  the  crocodile;  more 


generally  in  layers,  as  in  hoofs  and  nails,  and  also  in  the 
scales  of  fishes;*  and  occasionally  in  filaments,  as  in  hair; 

•  The  laminated  structure  of  the  scales  of  fishes  is  easily  distinguished  by- 
applying  to  them  a  high  magnifying  power.  As  the  breadth  of  each  new 
layer  is  greater  than  the  last,  its  edges  project  farther,  the  whole  surface 


ANIMAL  ORGANIZATION. 


93 


which  latter,  again,  are  often  agglutinated  together  by  a 
strong  cement,  uniting  them  into  a  hard  and  solid  structure, 
of  which  the  horn  of  the  rhinoceros  is  a  remarkable  exam- 
ple. In  all  cases,  the  portions  thus  successively  produced, 
are  no  longer  susceptible  of  being  nourished,  and  from  the 
moment  of  their  deposition,  undergo  no  farther  change,  ex- 
cept from  the  action  of  external  agents.  By  the  continual 
additions  that  are  made  to  them  at  their  base,  or  root,  where 
the  vessels  deposite  fresh  materials,  they  gradually  increase 
in  size,  protrude  through  the  skin,  and  continue  to  grow  by 
the  same  process  as  long  as  these  vessels  continue  in  acti- 
vity. 

The  nature  of  this  process  is  well  exemplified  in  the 
growth  of  hair.  Fig.  32  shows  the  apparatus  employed  in 
its  construction,  in  an  imaginary  section  of  the  root,  on  a 
magnified  scale.  Every  hair  takes  its  rise  from  a  minute 
vascular  pulp,  p,  of  an  oval  shape,  which  is  implanted  below 
the  corium,  or  true  skin,  p.*     This  pulp  is  invested  by  a 

sheath  or  capsule,  c,  which, 
together  with  the  contained 
pulp,  and  the  root  of  the  hair 
that  grows  from  it,  composes 
the  bulb  of  the  hair.  The 
bulb  itself  is  contained  in  a 
small  cell  formed  by  con- 
densed membranes,  s,  to 
which  it  has  no  attachment 
excepting  at  the  lower  part,  v, 
where  the  vessels  and  nerves 
of  the  pulp  are  passing  into 


having  that  concentric  striated  appearance  which  renders  it  an  interesting*  ob- 
ject for  microscopic  cxaniination.  Fig.  29  exhibits  the  striated  surface  of  the 
scale  of  the  Cyprinxis  alburnus,  and  Fig.  30  that  of  the  Perca  JluviatiUs. 
The  imbricated  arrangement  of  these  scales,  resembling-  that  of  the  tiles  on 
the  roof  of  a  house,  is  shown  in  Fig.  31.  All  these  figures  represent  the 
objects  highly  magnified. 

*  In  the  above  figure  r.  is  a  section  of  the  Epidermis,  or  cuticle;  the  dotted 
part,  R,  represents  tiic  situation  of  the  subjacent  retc  mucosum,  and  d,  the 
derm,  or  corium. 


94  THE  MECHANICAL  FUNCTIONS. 

it.  The  hair,  growing  by  depositions  from  the  inside  of  the 
capsule,  which  forms  the  outer  part,  o,  of  the  shaft,  and  from 
the  outside  of  the  pulp,  w^hich  forms  the  inner  or  central 
part,  I,  is  forced  upwards  till  it  has  pierced  the  skin;  and  in 
the  course  of  its  passage  a  canal  is  formed  for  it  in  the  skin 
itself,  and  continuous  with  that  which  encloses  the  bulb: 
and  the  course  of  this  canal  is  generally  oblique.  In  the  ele- 
phant, where  the  thickness  and  density  of  the  hide,  present 
considerable  obstacles  to  the  passage  of  the  hairs  through  it, 
we  may  discover,  on  minute  examination,  many  hairs  that 
have  only  penetrated  a  certain  way,  as  shown  at  b,  without 
ever  succeeding  in  reaching  the  surface. 

An  opinion  has  been  very  commonly  entertained  that 
each  hair,  on  its  protruding  from  underneath  the  cuticle, 
E,  at  the  point  q,  carries  up  along  with  it  a  portion  of  this 
outer  integument,  which,  stretching  as  the  hair  increases  in 
length,  forms  over  it  a  very  fine  external  tunic.  But  later 
observations  have  shown  that  this  is  not  the  case,  and  that 
there  is  simply  an  adhesion  of  the  edge  of  the  cuticle  to  the 
origin  of  the  hair,  without  any  accompanying  prolongation; 
so  that  if  the  whole  bulb  be  destroyed,  and  its  pulp  absorbed, 
the  hair  may  be  detached  by  the  slightest  force. 

From  this  account  it  will  be  seen  that  a  hair  is,  in  its  ori- 
gin, tubular;  the  inner  part  being  occupied  by  the  pulp. 
But  as  the  pulp  extends  only  to  that  portion  of  the  hair 
which  is  in  a  state  of  growth,  it  never  rises  above  the  sur- 
face of  the  skin;  and  the  cavity  in  the  axis  of  the  hair  is 
either  gradually  obliterated,  or  is  filled  with  a  dry  pith,  or 
light  spongy  substance,  probably  containing  air.  After  a 
certain  period,  the  bulb  diminishes  in  size,  from  the  collapse 
of  the  vessels  whose  powers  of  supplying  nutriment  become 
exhausted.  The  first  deficiency  in  its  nourishment  appears 
in  the  cessation  of  the  deposite  of  colouring  matter,  and  the 
hair  in  consequence  becomes  gray.  After  a  time,  the  ves- 
sels becoming  quite  impervious,  the  bulb  shrivels,  the  hair 
is  detached,  and  the  canal  which  its  root  occupied  in  the 
skin  becomes  obliterated. 


ANIMAL  ORGANIZATION.  95 

The  hair  of  difTcrcnt  animals,  and  even  of  different  parts 
of  the  same  animal,  is  very  various  in  its  shape,  texture,  and 
mechanical  properties.  Sometimes,  instead  of  being  cylin- 
drical, the  filaments  are  more  or  less  flattened,  striated, 
deeply  grooved,  or  even  beaded.  Instead  of  being  solid, 
they  may  even  be  tubular:  and  they  exhibit  also  the  great- 
est diversity  in  their  length,  fineness,  tenacity,  rigidity,  and 
disposition  to  curl.  All  these  varieties  may  be  traced  to 
corresponding  differences  in  the  form  and  the  relative  actions 
of  the  component  parts  of  the  bulb,  namely,  the  pulp  and  its 
capsule.* 

The  structure  of  the  organs  b}^  which  hairs  are  formed  is 
not  easily  distinguished,  in  the  ordinary  kinds  of  hair,  on 
account  of  their  minuteness:  it  is  readily  seen,  however,  in 
the  large  whiskers  of  the  feline  species,  and  also  of  the  seal, 
which  arc  subservient  to  more  extended  uses  than  those  of 
merely  covering  the  body,  and  which  are  even  supplied 
with  nerves,  converting  them  into  instruments  of  a  sense  of 
touch. 

In  the  quills  of  the  porcupine  a  still  more  complicated  or- 
ganization has  been  detected.  Fig.  33  shows  a  quill  with 
its  bulbous  root,  detached  from  the  body;  and  Fig.  34,  a 
transverse  section  magnified.  The  bulb  itself  is  contained 
in  a  distinct  cell,  shown  at  a.  Fig.  35,  which  represents  a 
longitudinal  section  of  these  organs.  This  cell  contains  a 
portion  of  fat  in  which  the  numerous  vessels  supplying  its 
pulp  and  capsule  are  embedded.  The  bulb  is  itself  sur- 
rounded by  an  outer  sheath,  s,  into  the  cavity  of  which,  b, 
there  opens  a  duct,  d,  proceeding  from  a  small  cell  or  fol- 
licle, F,  lodged  in  the  cellular  substance  on  the  outside  of 
the  sheath.  This  upper  cell  communicates  below  with  ano- 
ther cavity,  c,  containing  an  unctuous  matter.  During  the 
formation  of  the  quill  this  unctuous  matter  is  supplied  through 
this  channel,  and  probably  enters  as  an  ingredient  in  its 
composition.     The  capsule  of  the  pulp  consists  of  two  mem- 

*  See  F.  Cuvier's  Memoir  on  the  Formation  of  the  Quills  of  the  Porcu- 
pine, in  the  Nouvelles  Annulcii  du  Museum,  I.  429. 


96 


ANIMAL  ORGANIZATION. 


branes,  the  one  enveloping  the  other.  Fig.  36  shows  the 
bulb  laid  open  by  dividing  the  membranes  and  turning  them 
aside.     The  horny  portion  of  the  quill  is  secreted  by  the 


internal  membrane,  i,  and  deposited  in  successive  laminae. 
The  external  membrane  is  seen  at  o.  The  pulp  itself,  seen  at 
p,  is  still  more  curiously  organized;  its  surface  being  fluted, 
or  formed  into  longitudinal  processes.  The  horny  matter, 
being  deposited  on  these  processes,  is  moulded  to  their  shape, 
and  concretes  into  laminae  which  converge  from  the  circum- 
ference of  the  cylinder  towards  the  centre.  The  section 
(Fig.  36)  shows  these  converging  laminae,  which  being  of  a 
dark  colour,  give  to  the  surface  of  the  quill  the  appearance 
of  being  grooved;  this,  however,  is  merely  an  optical  illu- 
sion occasioned  by  the  dark  laminae  being  seen  through  the 
transparent  exterior  covering;  as  may  readily  be  detected 
by  viewing  the  surface  with  a  magnifying  glass.*  After  a 
certain  period  of  the  growth  of  the  quill,  the  pulp  ceases  to 
supply  the  materials  for  forming  the  spongy  substance  which 
occupies  the  interior  of  the  quill.  But  although  it  no  longer 
secretes,  it  still  retains  Its  place;  and  the  capsule  continuing 
to  deposite  horn,  the  quill  becomes  a  hollow  tube  of  consi- 
derable diameter.     When  it  has  attained  a  certain  size,  the 

•  It  is  observed  by  F.  Cuvler,  that  this  striated  appearance  is  peculiar  to 
the  quills  of  porcupines  of  the  old  world.  Those  from  America  have  no 
such  arrang-ement  of  laminae. 


MUSCULAR  POWER.  97 

pulp  begins  to  shrink,  and  the  diameter  of  the  tube  dimi- 
nishes; so  that  it  exhibits  a  tapering  form  at  both  ends.  Thus, 
mere  variations  in  the  bulk  and  the  action  of  the  pulp,  ac- 
companied with  changes  in  that  of  the  capsule,  are  sufficient 
to  account  for  every  diversity  in  the  form  and  condition  of 
the  resulting  structures. 

Among  the  mechanical  uses  of  the  integument,  that  of 
serving  as  a  cushion  for  relieving  the  more  prominent  parts 
of  the  frame,  and  especially  of  the  bones,  from  unequal  pres- 
sure, ought  not  to  be  overlooked.  This  object  is  promoted 
by  the  interposition  of  a  layer  oi  fat,  which  is  another  ani- 
mal substance  entitled  to  be  enumerated  among  the  elements 
of  its  structure.  It  consists  of  an  oily  fluid,  composed,  ac- 
cording to  the  analysis  of  Chevreuil,  of  two  constituent  prin- 
ciples, which  he  has  distinguished  by  the  terms  .s/eflfr2?ie  and 
elaine.  In  warm-blooded  animals  the  temperature  of  the 
body  is  always  sufficient  to  preserve  this  compound  sub- 
stance in  a  fluid  form:  but  it  is  prevented  from  being  dif- 
fused through  the  cellular  texture  by  being  contained  in 
separate  vesicles  of  extreme  minuteness.*  lience,  the  whole 
mass  of  the  fat,  which  is  thus  formed  of  an  aggregation  of 
these  vesicles,  has  not  the  appearance  of  being  fluid,  but 
seems  to  be  composed  of  small  grains  united  by  membranous 
investments  into  larger  masses;  a  structure  peculiarly  adapted 
to  the  purposes  of  a  soft  cushion,  retaining  only  a  small 
share  of  elasticity,  and  yielding  only  in  a  certain  limited 
degree  to  pressure. 

§  5.  Muscular  Power, 

In  Machines  contrived  by  human  skill  the  chief  art  con- 
sists in  devising  expedients  for  regulating  and  directing  the 
giving  moving  power,  so  that  it  may  bear,  in  the  proper 
degree,  and  in  the  proper  order,  upon  some  particular  ob- 
jects, and  produce  some  particular  effect.  The  whole  of  the 
apparatus  employed  with  this  intention,  however  numerous 

•  Dr.  Monro  estimated  their  diameter  at  between  the  800th  and  600th  of 
an  inch.     But  their  size  varies  in  different  animals. 

Vol.  I.  13 


98  THE  MECHANICAL  FUNCTIONS. 

may  be  its  parts,  however  various  the  forms  of  its  v»^heels, 
its  levers,  or  its  pulleys,  and  however  complicated  may  be 
their  connexions,  resolves  itself  into  a  series  of  intermediate 
instruments  for  the  transference  of  motion  from  the  source 
of  power,  or  the  point  v.'here  its  action  is  impressed,  to  the 
parts  which  are  designed  ultimately  to  receive  the  action  of 
the  force  employed.  It  is  an  established  principle  in  physics, 
that  mere  m.achinery  is  incapable  of  generating  mechanical 
force,  and  that  such  force  must  always  be  originally  derived 
from  some  extraneous  source.  Some  impulse  from  without, 
whether  it  be  the  pressure  of  the  wind,  the  fall  of  a  stream  of 
water, or  the  action  of  men  or  horses,  or  any  other  kind  of  fo- 
reign agency,  must  be  resorted  to,  both  to  set  the  engine  in 
motion,  and  to  continue  its  movements  when  they  are  once 
begun.  Nor  is  the  case  essentially  different  when  the  source 
of  motion  apparently  resides  in  some  internal  part  of  the 
machine  itself;  in  a  watch,  for  instance,  which  is  actuated  by 
the  main  spring;  or  in  a  steam-engine,  which  is  set  in  mo- 
tion by  the  elastic  vapour  contained  in  its  cylinder:  the 
spring  in  the  one  case,  and  the  vapour  in  the  other,  although 
they  may  in  one  sense  be  regarded  as  impelling  powers,  are, 
in  reality,  but  intermediate  agents  in  the  distribution  of  a 
force  orioiinatingfrom  other  sources.  In  the  watch,  the  force 
may  be  traced  to  the  hand  which  coiled  the  spring:  in  the 
steam-engine  to  the  fire,  which  has  imparted  elasticity  to 
the  vapour. 

The  living  body  differs  from  inorganic  machinery  in  con- 
taining within  itself  a  principle  of  motion  not  referrible,  as 
far  as  we  can  perceive,  to  any  of  the  primiary  forces  which 
exist  in  the  inanimate  world.  This  principle  has  been 
termed  conlractility.  In  animals  of  the  simplest  construc- 
tion, every  part  of  the  substance  of  the  body  seems  to  be 
equally  endowed  with  this  contractile  property,  although  ex- 
hibiting no  distinct  appearance  of  a  fibrous  structure.  This 
is  the  case  with  all  the  lower  zoophytes,  such  as  the  Infuso- 
ria, Polypi,  Medusas,  and  the  simpler  kinds  of  Eniozoa, 

Among  the  Polypi  and  Infusoria  we  meet  with  a  singular 
mode  of  acting  upon  the  surrounding  fluid  by  means  of  very 


MUSCULAR  POWER.  99 

minute  and  generally  microscopic  filaments,  which  the  ani- 
mal, by  some  unknown  power,  causes  to  vibrate  with  great 
rapidity.  Occasionally,  these  organs  are  found  even  in  ani- 
mals belonging  to  the  higher  classes.  Wherever  they  arc 
met  with,  they  perform,  as  will  hereafter  be  shown,  very 
important  functions;  sometimes  assisting  in  respiration,  at 
other  times  contributing  to  the  supply  of  food,  and  very  ge- 
nerally serving  as  instruments  of  progressive  motion. 

In  animals  placed  a  little  higher  in  the  scale,  we  begin  to 
trace  the  formation  of  fibres,  which  at  first  are  irregularly 
scattered  through  the  soft  substance:  but  as  the  organization 
becomes  more  refined,  these  fibres  are  collected  into  l)undles, 
and  compose  what  are  properly  called  m/iiscles.  Muscular 
fibres  are  attached  at  their  extremities  to  the  parts  intended 
to  be  moved.  In  the  lower  animals,  these  attachments  are 
principally  to  the  skin,  or  other  external  pnrts,  v/hich  arc 
subservient  to  the  purposes  of  progressive  motion.  In  the 
higher  classes,  the  solid  parts,  or  skeleton,  being  disposed 
more  in  the  centre  of  the  system,  the  niuscles  are  applied  to 
them  in  the  interior  of  the  body,  and  are  more  distinctly 
separated  into  masses,  each  having  its  proper  function  in 
the  movements  of  the  frame. 

The  peculiar  property  which  characterizes  the  muscular 
fibre  is  that  of  suddenly  shortening  itself,  so  as  to  bring  its 
two  ends,  and  the  parts  to  which  those  ends  are  connected, 
nearer  to  one  another.  This  contraction  is  performed  with 
astonishing  quickness  and  force,  and  the  accumulated  effect 
of  a  large  collection  of  these  fibres,  such  as  constitutes  a  mus- 
cle, is  therefore  capable  of  overcoming  great  resistances,  or 
of  raising  enormous  weights.  Those  muscles,  which,  by 
means  of  their  nerves,  as  will  hereafter  be  noticed,  are  sub- 
servient to  voluntary  motion,  are  excited  into  action  by  an 
exertion  of  the  will  of  the  animal.  There  are,  however,  a 
great  number  of  other  muscles,  the  contractions  of  which 
are  involuntary,  that  is,  are  produced  by  other  causes  than 
the  will.* 

*  These  two  classes  of  muscles  do  not  differ  in  their  outward  appearance: 
but  Dr.  Hodgkin  has  lately  pointed  out  a  curious  difference  in  tJie  micro- 


100  THE  MECHANICAL  PUNCTIONS. 

Muscular  contractility,  of  which  there  exists  no  trace  in 
the  vegetable  kingdom,*  has  been  established  by  nature  as 
the  primary  moving  power  of  the  animal  machine.  This 
agent  is  resorted  to  on  all  occasions  where  considerable  me- 
chanical force  is  wanted;  just  as  in  a  great  manufactory, 
where  an  immense  quantity  of  machinery  is  to  be  set  in  mo- 
tion, and  a  great  variety  of  work  is  to  be  executed,  the  hu- 
man mechanist  avails  himself  of  some  constant  moving  force, 
such  as  that  of  water,  or  steam.  The  laws  of  inorganic  mat- 
ter furnish  no  power  that  could  conveniently  have  been  ap- 
plied in  the  animal  body  for  that  purpose;  but  muscular 
power,  from  its  high  intensity,  is  adequate  to  every  object, 
and  has  been  accurately  adjusted,  by  the  most  refined  appli- 
cation of  the  laws  of  mechanism,  to  all  the  degrees  and  kinds 
of  effects  intended  to  be  produced. 

Although  the  power  be  the  same,  yet  the  mode  of  its  ap- 
plication is  exceedingly  diversified;  and  the  comparison  of 
these  diversities  is  the  more  interesting,  inasmuch  as  there 
are  few  of  the  animal  functions  in  which  the  ends  to  be  an- 
swered are  so  definite,  and  the  operation  of  the  expedients 
employed  is  so  plain  and  intelligible.  For  while  the  intri- 
cate chemical  processes  of  the  living  system  generally  elude 
our  research,  and  the  higher  faculties  of  sensation  and  per- 
ception are  dependent  on  still  more  recondite  and  mysteri- 
ous powers  of  nature,  the  mechanical  functions  are  effected 

scopic  structure  of  the  fibres  of  some  of  the  involuntary  muscles.  See  Ap- 
pendix to  his  Translation  of  Edwards  on  the  Influence  of  Physical  Ag-ents  in 
Life,  p.  443. 

*  The  principal  instances,  which  have  been  adduced  in  support  of  the 
opinion  that  muscularity  occasionally  exists  in  vegetable  structures,  are  the 
alternate  movements  of  the  leaflets  of  the  Hedysarum  gyrans,  which  have 
been  fancifully  compared  to  the  movements  of  the  ribs  in  respiration;  the 
quick  motions  of  the  stamina  of  the  Berberis,  Opuntia,  and  many  plants  of 
the  genera  Carduus,  and  Centaurea;  the  closing  of  the  leaves  of  the  Dionaea 
muscipula;  and  the  shrinking  of  those  of  the  Mimosa pudica,  or  sensitive 
plant.  On  a  superficial  view,  it  must  be  acknowledged  that  these  motions 
bear  a  resemblance  to  the  effects  of  muscular  contractility;  but  I  believe 
that  naturalists  are  now  generally  agreed  that  there  is  no  real  analogy  be- 
tween these  phenomena,  and  that  there  is  no  substantial  evidence  for  tlie 
existence  of  that  property  in  the  vegetable  kingdom. 


MUSCULAR  POWER. 


101 


by  the  simpler  properties  of  matter,  and  allow  us  a  clearer 
insight  into  the  wonderful  art  which  has  been  exerted  in 
their  accomplishment. 

Muscles,  during  their  contraction,  increase  in  thickness 
in  the  same  proportion  as  they  diminish  in  length. "^  It  is  on 
this  account,  more  especially,  that  a  knowledge  of  anatomy 


37 


38 


43 


40 


is  so  necessary  to  the  painter  and  the  sculptor.  In  every 
movement  and  attitude  of  the  body,  some  particular  sets  of 
muscles  are  in  action,  and  consequently  tense  and  prominent 
while  others  are  relaxed  and  flattened;  differences  which  it 
is  requisite  that  the  artist  should  faithfully  express,  in  order 
to  give  a  correct  representation  of  the  living  fio-ure. 

The  dilatation  of  the  muscular  fibres  in  thickness,  which 
accompanies  their  contraction  in  length,  would,  if  these 
fibres  had  been  loose  and  unconnected,  have  occasioned  too 
great  a  separation  and  displacement,  and  have  impeded  their 
co-operation  in  one  common  effect.  Nature  has  guarded 
against  this  evil  by  collecting  a  certain  number  of  the  ele- 
mentary fibrils,  and  tying  them  together  with  threads  of 
cellular  substance;  thus  forming  them  into  a  larger  fibre;  and 
again  packing  a  number  of  these  fibres  into  larger  bundles: 
always  surrounding  each  packet  with  a  web  of  cellular  tis- 

*  This  is  illustrated  by  the  annexed  figures,  37  and  38,  the  former  show- 
ing  tlie  relaxed  and  elongated,  and  the  latter  the  contracted  and  swollen 
state  of  the  same  muscle. 


102  THE  MECHANICAL  FUNCTIONS. 

sue;  which  thus  forms  a  separate  investment  for  each.  This 
plan  of  successive  reunion  into  larger  and  larger  assemblages 
is  carried  on  through  several  gradations  of  size,  till  the  en- 
tire muscle  is  completed. 

That  we  may  be  the  better  able  to  appreciate  the  excel- 
lence of  the  plans  adopted  in  the  mechanism  of  the  animal 
frame,  let  us  inquire  what  arrangements  would  occur  to  us, 
prior  to  an  acquaintance  with  those  actually  adopted,  as  the 
most  advantageous  dispositions  of  the  muscular  power.  It 
is  evident,  that  the  simplest  mode  would  be  that  of  extend- 
ing the  fibres  of  the  muscle  in  a  straight  line  between  the 
points  intended  to  be  brought  nearer  to  each  other.  This 
direct  application  of  the  power,  however,  is  seldom  compa- 
tible with  convenience,  unless  the  parts  to  be  moved  are  of 
very  small  size,  and  require  very  delicate  adjustments. 
Straight  muscles,  accordingly,  are  employed  chiefly  for  the 
movements  of  the  minuter  parts  of  the  apparatus  belonging 
to  the  senses,  such  as  the  eye,  and  the  ear,  and  also  that  of 
the  voice.  In  insects,  when  the  hard  case,  or  skeleton,  is 
wholly  external,  this  direct  application  of  the  moving  force 
is  also  very  generally  employed.  The  shells  of  the  bivalve 
mollusca,  as  of  the  Oyster  and  the  Cardhim,  are  closed  by 
one  or  two  straight  muscles,  the  fibres  of  which  pass  imme- 
diately from  the  inner  surface  of  the  one  to  that  of  the  other. 

In  the  greater  number  of  cases  it  is  more  convenient  to 
place  the  muscle  in  a  situation  which  causes  it  to  act  ob- 
liquely with  respect  to  the  direction  of  the  motion  produced 
in  the  part  to  which  it  is  attached.  This  will,  of  course,  be 
attended  with  a  loss  of  force  corresponding  to  the  degree  of 
obliquity;  but  there  are,  at  the  same  time,  advantages  gained, 
both  in  point  of  velocity  of  motion,  and  also  in  the  effect 
being  produced  by  a  smaller  extent  of  contraction  in  the 
fibres  of  the  muscle.  Oblique  muscles  are  frequently  em- 
ployed in  pairs,  and  are  made  to  act  on  opposite  sides  of  the 
line  of  the  intended  motion,  which  is,  in  this  case,  the  dia- 
gonal between  the  direction  of  the  two  equal  forces.  Thus, 
in  order  to  bring  a  bone  at  p.  Fig.  39,  down  to  the  point  q, 
the  two  muscles  a  and  b,  extending  from  the  fixed  points 


MUSCULAR  POWER.  103 

M  and  N,  may  be  employed;  for  as  they  exert  forces  in  the 
directions  p  m  and  p  n,  there  will  result  a  force  in  the  inter- 
mediate direction  p  o:  and  the  effect  desired  will  be  accom- 
plished more  quickly,  and  with  a  smaller  extent  of  contrac- 
tion in  the  muscles  producing  it,  than  if  the  same  power  had 
been  applied  by  means  of  a  straight  muscle  in  the  direction 
p  o.*  It  is  by  means  of  two  sets  of  muscles,  acting  thus  ob- 
liquely, that  the  ribs  are  brought  in  closer  approximation 
every  time  that  the  chest  is  elevated  in  breathing.  Thu» 
carefully  does  nature  dispose  the  muscular  fibres  so  as  to 
obviate  the  necessity  of  their  being  contracted  beyond  a  cer- 
tain extent:  and  thus  does  she  economize,  as  much  as  possi- 
ble, the  expenditure  of  muscular  power,  wherever  there  is 
a  constant  call  for  its  exertion. 

The  principle  which  I  have  just  explained,  whereby  cer- 
tain advantages  result  from  the  obliquity  of  the  action  of 
muscular  fibres,  is  applied,  not  only  to  the  entire  muscle, 
but  also  to  the  internal  arrangement  of  its  fibres.  Thus, 
we  generally  find  that,  in  a  flat  muscle,  its  upper  and 
under  surfaces  are  covered  by  a  thin  sheet  of  fibrous  tex- 
ture, or  thin  expansion  of  ligament  or  tendon;  and  that 
the  muscular  fibres  which  are  attached  to  them  are  direct- 
ed obliquely  from  the  one  to  the  other,  in  the  manner  re- 
presented by  the  section,  Fig.  40.  There  is  frequently  a 
middle  tendinous  layer  interposed  between  those  that  are  on 
the  surface  (as  shown  in  Fig.  41,)  in  which  case  the  muscu- 
lar fibres  pass  obliquely  from  the  former  to  the  latter,  but  in 
different  directions  on  each  side;  like  the  fibres  proceedino- 
from  the  shaft  of  a  pen.  A  muscle  thus  constructed  has  ac- 
cordingly been  termed  di  penniform  muscle;  as  is  exempli- 
fied in  the  straight  muscle  inserted  into  the  knee-pan  (the 
rectus  extensor  c?'uris,)  and  also  in  the  muscle  which  bends 
the  great  toe  (the  flexor  pollicis  jjedis  lo?igus.)  The  ar- 
rangement first  described.  Fig.  40,  forms  the  se?7ii-pcfi}ii- 
form  muscle;  an  instance  of  which  occurs  in  the  muscle  of 
the  leg,  which  is  termed  the  semimembranosus.     Frequent- 

•  See  a  paper  by  Dr.  Monro,  iji  the  Transactions  of  the  Koyal  Society  of 
Edinburgh.     Vol.  iii.  p.  250. 


104  THE  MECHANICAL  FUNCTIONS. 

ly  the  structure  is  rendered  still  more  complex,  by  the  in- 
terposition of  several  tendinous  layers  among  the  fleshy 
fibres.  This  arrangement,  which  constitutes  a  complex  tuus- 
cle,  (as  shown  in  Fig.  42)  occurs,  for  example,  in  the  Soiasus, 
or  large  muscle,  which  raises  the  heel,  and  forms  the  thick- 
est part  of  the  calf  of  the  leg. 

It  very  commonly  happens  in  the  animal  frame,  as  it 
does  in  other  machines,  that  the  presence  of  the  moving 
agent  in  the  spot  where  its  action  is  w^anted,  would  be  ex- 
ceedingly inconvenient.  The  usual  plan  adopted  for  trans- 
ferring the  effect  of  the  moving  power  to  a  distant  point  is 
the  employment  of  a  rope,  or  strap.  Such  is  precisely  the 
office  of  the  tendons,  which  are  long  straps,  attached  at  one 
end  to  the  muscle,  and  at  the  other  to  the  bone,  or  other 
part  intended  to  be  moved.  (See  Fig.  43.)  If  the  hand, 
for  instance,  had  been  encumbered  with  all  the  muscles 
which  are  necessary  for  the  movements  of  the  fingers,  it 
never  could  have  performed  its  office  as  a  delicate  mechani- 
cal instrument.  These  muscles,  accordingly,  are  disposed 
high  up  on  the  arm,  and  their  tendons  are  made  to  pass 
along  the  wrist  to  the  joints  of  the  fingers  which  .are  to  be 
moved. 

The  employment  of  tendons  is  accompanied  with  this 
farther  advantage,  that  by  their  intervention  the  united  pow- 
er of  all  the  fibres  of  the  muscle  may  be  obtained,  and  con- 
centrated upon  any  particular  point.  In  this  respect,  like- 
wise, they  resemble  a  rope,  at  which  a  great  number  of  men 
are  pulling  at  the  same  moment,  and  whose  combined  strength 
is  thus  brought  into  action.  Another  principal  use  of  ten- 
dons is  that  a  different  direction  may,  by  their  means,  be 
given  to  the  moving  power,  without  altering  its  position. 
Many  instances  occur  of  their  application  in  this  manner, 
by  their  being  made  to  pass  round  corners  of  bones,  and 
along  grooves,  or  channels,  expressly  formed  for  their  trans- 
mission, and  producing  the  effect  of  pulleys. 

In  a  great  number  of  muscles,  the  fibres,  instead  of  running- 
parallel  to  one  another,  are  made  either  to  converge,  or  to 
diverge,  in  order  to  suit  particular  kinds  of  movements:  and 


MUSCULAR  POWER. 


105 


wc  frequently  find  that  different  portions  of  the  same  mus- 
cle have  the.  power  of  contracting  independently  of  the  rest, 
so  as  to  be  capable  of  producing  very  various  effects,  accord- 
ing as  they  act  separately  or  in  combination.  This  is  exem- 
plified in  the  muscle  of  the  back,  called  the  Trapezius,  repre- 
sented in  Fig.  44.  In  many  instances,  the  fibres  radiate  in  all 
directions  from  a  common  centre:  this  is  the  case  with  the 
delicate  muscle  of  the  ear-drum,  as  shown  in  Fig.  45.  In 
that  of  the  elephant,  which  is  about  an  inch  and  a  half  in 
diameter,  these  radiating  fibres  are  very  conspicuous,  even 
to  the  naked  eye:  and  they  are  also  visible  in  the  membrane 
of  the  human  ear,  when  viewed  with  a  good  microscope.* 

At  other  times,  the  muscular  fibres  run  in  a  circular  di- 
rection, forming  what  is  called  an  orbicular,  or  sphincter 
muscle,  of  which  an  example  occurs  in  that  which  surrounds 
and  closes  the  eye.  (Fig.  4G.)  Very  frequently  these  two 
last  modes  of  arrangement  are  united  in  some  part,  as  ap- 
pears to  be  the  case  in  the  membrane  of  the  eye,  called  the 
Iris.  (Fig.  47.)  The  circular  fibres  of  the  iris  surround  the 
central  aperture,  or  pupil,  the  size  of  which  they  diminish 
when  they  contract;  while,  on  the  contrary,  the  radiating 
fibres,  acting  on  the  inner  circle,  and  drawing  it  nearer  to 
the  outer  circumference,  which  is  fixed,  lessen  the  breadth 
of  the  ring,  and  consequently  enlarge  the  circular  aperture. 


47 


43 


A  similar  combination  of  radiating  and  circular  fibres  is 
employed  in  the  construction  of  flat,  or  slightly  concave 
muscular  disks,  which  are  thus  rendered  capable  of  exerting 
a  strong  force  of  adhesion  to  the  surfaces  on  which  they  are 
applied.     In  these  organs  the  circular  fibres  are  placed  at 


Vol.  I. 


Home  Phil.  Trans,  for  1800,  p.  1. 
14 


106 


THE  MECHANICAL  FUNCTIONS. 


the  circumference,  and  the  radiating  fibres  in  the  interior  of 
the  sucker,  (see  Fig.  48;)  so  that,  while  the  margin  of  the 
disk  is  closely  applied  to  the  object,  the  force  resulting  from 
the  contraction  of  the  circular  fibres  is  exerted  to  remove 
the  central  portions  from  the  surface  of  attachment,  and 
thereby  tends  to  create  a  vacuum  underneath  the  disk;  the 
two  surfaces  remain,  therefore,  strongly  attached  by  the  at- 
mospheric pressure,  which  acts  on  their  outer  sides.  An  ap- 
paratus of  this  kind,  as  we  shall  afterwards  find,  is  met  with 
very  frequently  among  the  lower  orders  of  the  animal  king- 
dom. 


Another  kind  of  circular  disposition  of  fibres  is  that  which 
occurs  in  the  muscular  coats  surrounding  canals  of  various 
kinds,  such  as  the  blood  vessels  and  the  alimentary  tube. 
Their  action  tends  to  contract  the  diameter  of  the  canal,  and 
to  exert  pressure  on  its  contents.  In  these  cases,  there  is 
2;enerally  at  the  same  time  provided  another  layer  of  fibres, 
disposed  longitudinally,  as  shown  in  Fig.  49;  the  circu- 
lar fibres  being  seen  in  Fig.  50.  The  action  of  the  longi- 
tudinal fibres  is  evidently  to  shorten  the  canal;  while  that 
of  the  circular  fibres,  by  the  yielding  and  the  partial  re- 
action of  the  contents  of  the  vessel,  has  a  tendency  to 
extend  it.  The  Jiscidia,  which  is  a  species  of  marine  worm, 
is  an  example  of  an  animal  whose  skin  contains  a  union  of 
straight  and  circular  fibres,  by  which  all  its  movements  are 
readily  performed.  Many  instances  occur  in  the  cylindrical 
envelopes  of  animals,  of  the  combination  of  a  third  series  of 
fibres,  passing  obliquely,  with  those  which  have  transverse 
and  longitudinal  directions.  In  the  muscular  skin  of  the 
Leechy  for  example,  besides  two  internal  layers  of  longitudi- 
nal fibres,  an  external  one  has  lately  been  discovered,  which 
is  composed  of  oblique  or  spiral  fibres,  crossing  one  another 


MUSCULAR  POWER.  107 

in   opposite  directions,  and  greatly  facilitating  the  varied 
movements  of  the  animal.* 

A  variety  of  still  more  complicated  arrangements  may  be 
traced  in  the  fibres  of  those  muscles  which  invest  hollow 
sacs,  or  receptjicles,  such  as  the  stomach,  (Fig.  51,)  and  the 
heart,  (Fig.  52.)  We  find,  in  the  substance  of  these  organs, 
sets  of  fibres,  which  pass  in  a  spiral  direction,  and  which, 
consequently,  unite  the  effects  of  both  longitudinal  and  cir- 
cular fibres;  and,  when  combined  with  either  of  these,  they 
serve  to  modify  and  regulate  the  actions  of  each  organ  in  a 
great  variety  of  ways.t 

The   infinite  mechanical  skill,   with  which  the  moving 
power  has  been  applied  to  the  purposes  to  be  accomplished, 
is  displayed  not  only  in  the  larger  organs,  where  great  force 
is  to  be  exerted,  but  also,  in  a  still  more  conspicuous  man- 
ner, in  the  execution  of  the  smaller  motions,  requiring  the 
most  accurate  regulation,  and  the  nicest  adjustments.     We 
cannot  but  be  struck  with  the  accordance  which  may  often, 
in  these  instances,  be  traced  with  human  contrivances,  when 
the  greater  motions  are  rapidly  executed  by  one  set  of 
agents,  acting  with  considerable  power  and  velocity,  while 
the  minuter  approximations  to  the  exact  positions  are  eflfect- 
ed  by  a  distinct  part  of  the  apparatus,  capable  of  more  deli- 
cate action,  though  with  a  smaller  force.     Thus,  while  the 
astronomer  brings  his  telescope  round  by  powerful  machi- 
nery, so  as  to  direct  it  to  that  part  of  the  heavens,  where 
the  object  he  wishes  to  view  is  situated,  a  more  nice  me- 
chanism is  employed  to  direct  the  instrument  accurately  to 
the  exact  point;  and,  again,  another  is  provided  for  making 
the  proper  focal  adjustments.     Many  parallel  cases  occur  in 

*  Carus,  Tabulae  Anat.  Comp.  fol.  Tab.  I.  Fig".  6. 

•j-  The  muscular  fibres  of  the  heart  are  disposed  in  two  layers;  each  set 
passing  in  a  spiral  course  from  the  basis,  or  broad  part,  to  the  point  or  apex; 
but  the  direction  of  the  turns  being-  different  in  each,  the  two  layers  cross  or 
decussate,  producing-  the  effect  and  procuring-  the  advantages  of  a  combina- 
tion of  oblique  muscles  already  explained.  Thus  beautifully  is  the  arrange- 
ment of  tiie  muscular  fibres  of  the  heart  calculated  to  produce  the  rapid  and 
complete  expulsion  of  its  contained  blood,  with  tlie  smallest  amount  of  con- 
traction in  the  individual  fibres. 


108  THE  MECHANICAL  FUNCTIONS. 

the  mechanism  of  the  animal  frame;  one  set  of  powerful 
muscles  being  employed  for  the  larger  movements,  and  ano- 
ther set  provided  for  the  accurate  regulation  of  the  more 
delicate  inflections  and  nicer  positions.  This  we  shall  find 
exemplified  in  the  movements  of  the  fingers,  and  of  many 
of  the  organs  of  the  finer  senses. 

In  general,  however,  we  may  observe  that,  the  mechanical 
expedients  devised  by  Nature  for  efiecting  each  particular 
purpose  are  characterized  by  the  most  admirable  simplicity. 
In  this  respect,  also,  as  well  as  in  all  others,  we  cannot  fail 
to  recognise  their  infinite  superiority  over  every  correspond- 
ing invention  of  man. 

"In  human  works,  thoug-h  labour'don  with  pain, 
A  thousand  movements  scarce  one  purpose  gain: 
In  God's,  one  single  can  its  ends  produce. 
Yet  serves  to  second,  too,  some  other  use."    Pope. 

We  may  generally  observe,  in  the  mechanism  of  the 
joints,  that  the  muscles  are  made  to  act,  either  directly  or 
by  means  of  their  tendons,  at  a  point  much  nearer  to  the 
axis  of  motion  than  the  resistance  to  be  overcome.  With 
regard  to  the  direct  force,  therefore,  it  is  evident  that  they 
must  act  with  a  great  mechanical  disadvantage;  and  this  dis- 
advantage is  still  farther  increased  by  the  obliquity  of  the 
action  with  reference  to  the  direction  of  the  motion.  But 
the  contractile  power,  which  is  inherent  in  the  muscular 
fibre,  is  so  enormous,  as  amply  to  afi'ord  these  losses,  great 
as  they  necessarily  are;  while,  on  the  other  hand,  full  com- 
pensation is  made  by  the  greater  freedom  and  velocity  of 
motion  thereby  obtained.  Strength  is  sacrificed  without 
scruple  to  beauty  of  form  or  convenience  of  purpose;  and 
that  disposition  of  the  force  is  always  adopted,  from  which, 
on  the  whole,  the  greatest  practical  benefit  results.  Every 
where  do  we  find  the  wisest  adaptation  of  muscular  power 
to  the  objects  proposed,  whether  it  be  exerted  in  laborious 
efforts  of  the  limbs  and  trunk;  whether  employed  in  ba- 
lancing the  frame,  or  urging  it  into  quick  progression;  or 
whether  it  be  applied  to  direct  the  delicate  evolutions  of 


MUSCULAR  POWER.  109 

the  fingers,  the  rapid  movements  of  the  organs  of  speech, 
or  the  more  exquisite  adjustments  of  the  eye,  or  of  the  in- 
ternal ear.  Amidst  the  endless  combinations  of  machinery 
exhibited  in  different  parts  of  the  animal  kingdom,  althou<rh 
the  mode  of  application  be  diversified  in  ten  thousand  ways, 
the  original  power  is  still  of  the  same  kind,  and  is  regulated 
by  the  same  physical  laws;  and  similar  instruments  are  em- 
ployed in  effecting  this  infinite  variety  of  purposes,  by  the 
all-wise  and  omnipotent  Architect  of  animated  creation. 


(     110     ) 


CHAPTER  II. 


THE  MECHANICAL  FUNCTIONS  IN  ZOOPHYTES. 

§  1.  General  Observations, 

The  mechanism  of  an  organized  being  is  designed  to 
fulfil  various  important  objects.  These  we  may  distinguish 
into  two  classes;  the  one  having  reference  to  its  internal 
welfare,  the  other  to  its  relations  with  external  bodies.  The 
diflferent  parts  of  its  system  must,  in  the  first  place,  be  me- 
chanically united  and  supported,  as  well  as  protected  from 
injurious  external  impressions;  and  they  must  at  the  same 
time  be  so  constructed  as  to  admit  of  all  the  internal  move- 
ments, which  the  performance  of  their  functions  renders 
necessary.  They  must,  in  the  second  place,  be  made  capa- 
ble of  exerting  upon  external  matter  the  actions  which  con- 
duce to  their  well  being;  and,  in  order  to  enlarge  their 
sphere  of  action,  they  must  have  the  power  of  transferring 
the  whole  body  from  one  place  to  another;  or,  in  other 
words,  of  effecting  its  progressive  motion. 
'  The  objects  included  in  the  first  of  these  branches  of  the 
mechanical  functions  are  answered  by  the  organization  both 
of  the  vegetable  and  the  animal  systems:  but  those  of  the 
latter  belong  exclusively  to  the  functions  of  animal  life. 
The  power  of  locomotion,  more  especially,  constitutes  the 
most  general  and  palpable  feature  of  distinction  between 
these  two  classes  of  beings.  A  plant,  during  the  whole  pe- 
riod of  its  existence,  is  fixed  to  the  spot  where  it  was  first 
produced,  and  is  dependent  for  the  continuance  of  its  life  on 
local  circumstances;  such  as  the  nature  of  the  soil  in  which 
its  roots  are  embedded,  and  the  qualities  of  the  air  and  wa- 


ZOOPHYTES.  Ill 

ter  in  its  immediate  vicinity.  It  is  exposed  so  the  action 
of  the  surrounding  elements,  and  affected  by  their  vicissi- 
tudes, without  the  means  of  retreat,  and  without  the  power 
of  reaction.  With  respect  to  all  external  agents,  indeed,  ve- 
getables may  be  regarded  as  passive  beings.  Very  different 
are  the  condition  and  destination  of  animals.  Excepting 
a  few  among  the  lower  orders  of  the  creation,  such  as  Zoo- 
phytes and  MoUusca,  all  animals  are  gifted  with  the  power 
of  spontaneously  changing  their  situation,  according  to  their 
several  wants  and  necessities,  and  are  thus  enabled  to  seek 
and  to  choose  those  objects  which  are  salutary,  and  to  avoid 
or  reject  those  which  are  injurious.  Nature  has,  for  these 
purposes,  furnished  them  with  a  more  complex  organization 
and  more  varied  powers,  adapted  to  a  greater  diversity  of 
pursuits,  and  to  a  higher  and  more  expanded  sphere  of  ex- 
istence. 

The  power  of  progressive  motion  is  enjoyed  in  very  dif- 
ferent degrees  by  different  races  of  animals,  according  to  the 
particular  model  on  which  they  are  constructed,  and  the  re- 
lations which  their  organization  bears  to  the  element  assigned 
as  their  residence.  All  the  mechanical  circumstances  in 
their  economy,  indeed,  are  so  closely  linked  together,  as 
scarcely  to  admit  of  being  considered  separately.  Thus,  we 
find,  in  one  animal,  a  variety  of  mechanical  effects  accom- 
plished by  one  and  the  same  instrument;  while,  in  others, 
they  are  each  produced  by  a  separate  and  distinct  organ.  In 
some,  the  leading  principle  of  the  construction  is  simplicity; 
in  others,  the  most  elaborate  mechanism  is  displayed.  But 
the  means  have  constant  reference  to  the  design,  and  are  ever 
varied  in  exact  conformity  with  the  change  of  purpose.  The 
relative  advantages  of  each  plan  of  structure  appear  to  have 
been  carefully  estimated,  and  studiously  balanced.  Each 
quality  has  been  bestowed  in  different  degrees  of  perfec- 
tion; so  that  in  following  the  series  of  gradation  among  the 
successive  tribes  of  animals,  we  occasionally  meet  with  fa- 
voured species,  endowed  with  great  superiority  in  some 
particular  faculty.  Some  animals  excel  in  swiftness;  others 
in  strength.  Some  are  qualified  to  dive  into  the  recesses  of 
the  deep;  others  to  flutter  in  the  light  regions  of  air;  while, 


112  THE  MECHANICAL  FUNCTIONS. 

in  many  of  the  inferior  ranks,  we  find  all  these  objects  re- 
nounced for  the  more  certain  advantage  of  security,  which 
the  softer  texture  of  the  organs  renders  one  of  paramount 
importance.  That  construction  of  limbs  which  favours  cer- 
tain movements  will  necessarily  interfere  with  the  ready 
performance  of  others,  and  must  preclude  the  development 
of  the  organs  which  would  be  necessary  for  facilitating  them. 
Different  kinds  of  prey  require  dexterity  in  particular  ac- 
tions for  their  pursuit  and  seizure.  The  animal  is,  in  one 
case,  formed  for  climbing  trees;  in  another,  for  burrowing 
in  the  earth:  in  a  third,  for  perforating  wood.  Some  are 
provided  with  organs  for  penetrating  into  the  bodies  of  other 
animals;  others  with  the  means  of  insnaring  their  captives; 
while  others,  again,  instil  into  the  veins  of  their  victims  a 
deadly  poison.  Hence  it  is  necessary,  in  studying  the  or- 
ganization of  animals,  to  bestow  particular  attention  on  the 
habits  and  mode  of  life  for  which  each  respective  tribe  and 
species  has  been  destined. 

In  the  examination  of  the  mechanical  functions  which  will 
form  the  first  part  of  this  treatise,  I  shall  keep  in  view,  as 
the  leading  object  of  inquiry,  the  faculty  oi p7'0gressive  ino- 
tion,  noticing  its  different  degrees  of  perfection  as  we  fol- 
low the  ascending  series  of  animals;  but  adverting,  also,  oc- 
casionally, to  the  other  topics  which  belong  to  this  class  of 
functions. 

It  may  be  observed  in  general,  that  the  mechanical  con- 
struction of  animals  which  constantly  inhabit  a  watery  ele- 
ment is  more  simple  than  the  construction  of  those  which  live 
on  land,  and  are  encompassed  by  a  lighter  medium.  Dif- 
fering but  little  in  their  specific  gravity  from  the  fluid  in 
which  they  are  immersed,  aquatic  animals  are  necessarily 
supported,  on  all  sides,  by  a  powerful  hydrostatic  pressure, 
which  nearly  balances  the  force  of  gravity,  and  counter- 
acts the  tendency  of  their  bodies  to  descend  in  the  fluid. 
Many  of  the  obstacles  to  progressive  motion  are  thus  re- 
moved; and  there  is  no  necessity  for  the  compactness  of 
frame,  and  the  rigidity  and  cohesion  of  substance  which  are 
required  in  terrestrial  animals. 


SPONGES.  113 

The  animals  that  occupy  the  lower  divisions  of  the  scale 
can  exist  only  in  a  liquid  element.  Their  forms  present 
many  analogies  with  vegetables;  and  hence  they  have  been 
denominated  Zooj)hytes,  that  is,  animated  plants:  but  as  it 
is  now  well  ascertained  that  they  possess  the  essential  cha- 
racters of  animals,  the  term  of  Phytozoa.  or  plant-like  ani- 
mals, which  has  been  given  to  them  by  some  modern  writers, 
would  appear  to  be  a  more  appropriate  designation.  It  is, 
however,  scarcely  worth  while  at  the  present  day,  to  change 
a  name  so  generally  received  as  that  of  Zoophytes,  and  the 
application  of  which  is  not  likely  to  lead  to  any  misunder- 
standing. 

§  2.  Porifera,  or  Sponges. 

Among  Zoophytes,  the  lowest  station  in  the  scale  of  or- 
ganization is  occupied  by  the  tribes  of  Porifera^  the  name 
given  by  Dr.  Grant  to  the  animals  which  form  the  various 
species  of  sponge,  and  which  are  met  with  in  such  multi- 
tudes on  every  rocky  coast  of  the  ocean,  from  the  shores  of 
Greenland  to  those  of  Australia.     Sponges  grow  to  a  larger 
size  within  the  tropics,  and   are  found  to  be  more  diminu- 
tive, and  of  a  firmer  texture,  as  we  approach  the  Polar  cir- 
cles.    Dr.  Grant  observes*  that  they  are  met  with  equally 
in  places  covered  perpetually  by  the  sea,  as  in  those  which 
are  left  dry  at  every  recess  of  the  tide.     They  adhere  to, 
and  spread  over  the  surface  of  rocks  and  marine  animals,  to 
which  they  are  so  firmly  attached  that  they  cannot  be  re- 
moved without  lacerating  and  injuring  their  bodies.     "Al- 
though they  thrive  best,"  he  farther  remarks,  "in  the  shel- 
tered cavities  of  rocks,  they  come  to  maturity  in  situations 
exposed  to  the  unbroken  fury  of  the  surge.     They  cover  the 
nakedness  of  cliffs  and  boulders;  they  line  with  a  variegated 
and  downy  fleece  the  walls  of  submarine  caves,  or  hang  in 
living  stalactites  from  the  roof. " 

In  their  general  appearance  they  resemble  many  kinds  of 

*  Edinburgh  Philosophical  Journal,  vol.  xiii.  p.  94. 
Vol.  I.  15 


114 


THE  MECHANICAL  FUNCTIONS. 


plants,  but  in  their  internal  organization  they  differ  entirely 
from  every  vegetable  production;  being  composed  of  a  soft 
flesh,  intermixed  with  a  tissue  of  fibres,  some  of  which  are 
solid,  others  tubular;  and  the  whole  being  interwoven  to- 
gether into  a  curious  and  complicated  net-work.  The  sub- 
stance of  which  this  solid  portion,  or  basis,  is  formed,  is 
composed  partly  of  Jiorn,  and  partly  of  siliceous  or  calcare- 
ous matter.  It  has  been  termed  the  axis  of  the  Zoophyte; 
and  as  it  supports  the  softer  substance  of  the  animal,  it  may 
be  regarded  as  performing  the  office  of  a  skeleton,  giving  form 
and  protection  to  the  entire  fabric. 

The  material  of  which  the  fleshy  portion  is  composed  is 
of  so  tender  and  gelatinous  a  nature  that  the  slightest  pres- 
sure is  sufficient  to  tear  it  asunder,  and  allow  the  fluid  parts 
to  escape;  and  the  whole  soon  melts  away  into  a  thin  oily 
liquid.  When  examined  with  the  microscope  the  soft  flesh 
is  seen  to  contain  a  great  number  of  minute  grains,  dissemi- 
nated through  a  transparent  jelly.  Every  part  of  the  sur- 
face of  a  living  sponge  (as  may  be  seen  in  Fig.  53)  presents 
to  the  eye  two  kinds  of  orifices;  the  larger  having  a  rounded 


55 


shape,  and  generally  raised  margins,  which  form  projecting 
papillse;  the  smaller  being  much  more  numerous,  and  ex- 
ceedingly minute,  and  constituting  what  are  termed  the 
pores  of  the  sponge. 

It  has,  for  a  long  time,  been  the  received  opinion  among 
naturalists  that  this  superficial  layer  of  gelatinous  substance 
was  endowed  with  a  considerable  power  of  contractility:  it 
was  generally  believed  that  it  shrunk  from  the  touch,  and 
that  visible  tremulous  motions  coulcl  be  excited  in  it  by 


SPONGES.  115 

punctures  with  sharp  Instruments,  or  other  modes  of  irrita- 
tion. It  is  extraordinary  that  errors  like  these  should  have 
crept  into  the  writings  of  modern  zoologists  of  the  highest 
authority,  such  as  Lamarck,  Bruguiere,  Gmelin,  Bosc,  and 
Lamouroux.*  The  notion  that  the  sponge  contracts  when 
touched  is  of  very  ancient  date,  for  it  may  even  be  traced 
beyond  the  time  of  Aristotle;  and  it  has  been  handed  down 
by  succeeding  naturalists,  and  echoed  from  the  one  to  the 
other,  so  as  to  have  gained  admission,  without  being  ques- 
tioned, in  all  the  recent  systematic  works  on  Zoology. 

The  alleged  spontaneous  palpitation  of  the  flesh,  occur- 
ring in  particular  parts,  had  its  origin  in  the  views  taken  of 
the  nature  of  sponges,  by  Marsigli,  an  Italian  naturalist,  who 
in  the  year  1771,  announced  that  he  had  seen  movements  of 
dilatation  and  contraction  in  the  round  apertures  visible  on 
the  surface  of  sponges.  This  statement,  so  confidently  ad- 
vanced, seems  to  have  made  a  strong  impression  on  Ellis, 
who,  while  pursuing  a  similar  train  of  observations,  came  to 
persuade  himself  that  he  could  see,  not  only  the  movem.ents 
described  by  Marsigli,  but  also  the  passage  of  water  to  and 
fro,  through  the  same  apertures.  He  communicated  this 
account  to  the  Royal  Society  in  1765;  it  was  published  in 
its  Transactions,!  and  will  ever  remain  an  instructive  proof 
of  the  degree  in  whicli  our  very  perceptions  may  be  influ- 
enced by  preconceived  views,  and  by  the  force  of  the  ima- 
gination. Pallas  immediately  admitted,  without  examina- 
tion, the  hasty  assertion  of  Ellis,  into  his  '•' Elenchiis 
ZoophytoruTYif^  whence  it  was  copied  by  succeeding  au- 
thors, and  the  error  became  at  length  so  widely  dissemi- 
nated, that  for  more  than  half  a  century  it  was  received  as  an 
established  fact  in  natural  history.  The  elaborate  and  accu- 
rate researches  of  Dr.  Grant  on  these  subjects  have  at  length 
dispelled  the  prevailing  illusion,  and  have  clearly  proved 
that  the  sponge  does  not  possess,  in  any  sensible  degree, 

•  This  mistaken  view  was  adopted  by  Cuvier  in  the  first  edition  of  his 
"Regne  Animal,"  T.  iv.  p.  88;  but  Dr.  Grant's  rectification  of  the  error 
is  noticed  in  the  second  edition  of  that  work. — T,  iii.  p.  322. 

t  Vol.  Iv.  p.  284. 


116  THE  MECHANICAL  FUNCTIONS. 

that  power  of  contraction  which  had,  for  so  many  ages,  been 
ascribed  to  it.* 

Dr.  Grant  has  also  shown  the  true  nature  of  the  currents 
of  fluid  issuing  at  difTerent  points  from  the  surface  of  these 
animals,  as  well  as  the  absence  of  all  visible  movements  in 
the  orifices  which  give  exit  to  the  fluid.  Never  did  he  find, 
in  his  experiments,  the  slightest  appearance  of  contraction 
produced  in  any  part  of  the  sponge,  by  puncturing,  lace- 
rating, burning,  or  otherwise  injuring  its  texture,  or  by  the 
application  of  corrosive  chemical  agents.  Of  his  discovery 
of  the  fluid  currents,  he  gives  the  following  interesting  ac- 
count: "  I  put  a  small  branch  of  the  Spongia  coalita,  with 
some  sea-water,  into  a  watch-glass,  under  the  microscope, 
and,  on  reflecting  the  light  of  a  candle  through  the  fluid,  I 
soon  perceived  that  there  was  some  intestine  motion  in  the 
opaque  particles  floating  through  the  water.  On  moving 
the  watch-glass,  so  as  to  bring  one  of  the  apertures  on  the 
side  of  the  sponge  fully  into  view,  I  beheld,  for  the  first  time, 
the  splendid  spectacle  of  this  living  fountain,  vomiting  forth, 
from  a  circular  cavity,  an  impetuous  torrent  of  liquid  matter, 
and  hurling  along,  in  rapid  succession,  opaque  masses,  which 
it  strewed  every  where  around.  The  beauty  and  novelty  of 
such  a  scene  in  the  animal  kingdom,  long  arrested  my  atten- 
tion, but  after  twenty-five  minutes  of  constant  observation, 
I  was  obliged  to  withdraw  my  eye  from  fatigue,  without 
having  seen  the  torrent  for  one  instant  change  its  direction, 
or  diminish,  in  the  slightest  degree,  the  rapidity  of  its  course. 
I  continued  to  watch  the  same  orifice,  at  short  intervals,  for 
five  hours,  sometimes  observing  it  for  a  quarter  of  an  hour 
at  a  time,  but  still  the  stream  rolled  on  w^ith  a  constant  and 
equal  velocity."  About  the  end  of  this  time,  however,  the 
current  became  languid,  and,  in  the  course  of  another  hour, 
it  ceased  entirely.  Similar  currents  were  afterwards  ob- 
served by  Dr.  Grant  in  a  great  variety  of  species.  They 
take  place  only  from  those  parts  that  are  under  water,  and 

•  See  his  papers  on  this  subject  in  the  Edinburgh  Philosophical  Journal, 
vol.  xiii.  p.  95  and  333,  fi"om  which  most  of  the  facts  mentioned  in  the  above 
account  are  taken. 


SPONGES.  117 

immediately  cease  when  the  same  parts  are  uncovered,  or 
when  the  anima^^dies. 

It  thus  appears  that  the  round  apertures  in  the  surface  of 
a  living  sponge  are  destined  for  the  discharge  of  a  constant 
stream  of  water  from  the  interior  of  the  body;  carrying 
away  particles,  which  separate  from  the  sides  of  the  canals, 
and  which  are  not  only  seen,  under  the  microscope,  con- 
stantly issuing  from  these  orifices,  but  may  even  be  per- 
ceived by  the  naked  eye,  propelled  occasionally  in  larger 
masses.* 

For  the  supply  of  these  constant  streams,  it  is  evident  that 
a  large  quantity  of  water  must  be  continually  i-eceived  inta 
the  body  of  the  sponge.  It  is  by  the  myriads  of  minute 
pores,  which  exist  in  every  part  of  the  surface,  that  this 
water  enters,  conveying  with  it  the  materials  necessary  for 
the  subsistence  of  the  animal.  These  pores  conduct  the  fluid 
into  the  interior,  where,  after  percolating  through  the  nu- 
merous channels  of  communication  which  pervade  the  sub- 
stance of  the  body,  it  is  collected  into  wider  passages,  ter- 
minating in  the  fecal  orifices  above  described,  and  is  finally 
discharged.  The  mechanism  by  which  these  currents  are- 
produced  is  involved  in  much  obscurity.  There  can  be  no» 
doubt  that  they  are  occasioned  by  some  internal  movements; 
and  the  analogy  of  other  zoophytes  would  lead  us  to  ascribe 
them  to  the  action  of  fibrils,  or  cilia,  as  they  are  termed,  pro- 
jecting from  the  sides  of  the  canals  through  which  the  streams 
pass;  but  these  cilia  have  hitherto  eluded  observation,  even 
with  the  highest  powers  of  the  microscope. 

The  organization  of  sponges  is  as  regular  and  determinate 
as  that  of  any  other  animal  structure,  and  presents  as  syste- 
matic an  arrangement  of  parts.  In  some  species,  such  as  the 
common  sponge,  the  basis  is  horny  and  elastic,  and  composed 
of  cylindric  tubes,  which  open  into  each  other,  and  thus  form 
continuous  canals  throughout  the  whole  mass. 

•  The  currents  issuing  from  the  larger  orifices  are  best  seen  by  placing-  the 
living  animal  in  a  shallow  vessel  of  sea-water,  and  strewing  a  little  powdered 
chalk  on  the  surface,  the  motions  of  which  will  render  the  currents  very 
sensible  to  the  eye.     Fig.  53  exhibits  these  phenomena. 


118  THE  MECHANICAL  FUNCTIONS. 

Others  have  a  kind  of  skeleton,  composed  of  a  tissue  of 
needle-shaped  crystals  of  carbonate  of  li^e,  or  of  silex. 
These  hard  and  sharp-pointed  fibres,  or  spicula,  are  disposed 
around  the  internal  canals  of  the  sponge,  in  the  order  best 
calculated  to  defend  them  from  compression,  and  from  the 
entrance  of  foreign  bodies.  Some  of  these  spicula  are  deli- 
neated in  Fig.  54:  but  their  forms,  although  constant  in  each 
species,  admit  of  considerable  diversity  in  the  difierent  kinds 
of  sponge. 

Although  sponges,  in  common  with  the  greater  number 
of  zoophytes,  are  permanently  attached  to  rocks,  and  other 
solid  bodies  in  the  ocean,  and  are  consequently  destined  to 
an  existence  as  completely  stationary  as  that  of  plants,  yet 
such  is  not  the  condition  of  the  earlier,  and  more  transitory 
stages  of  their  development.  Nature,  ever  solicitous  to  pro- 
vide for  the  multiplication  of  each  race  of  beings,  and  for 
.  their  dissemination  over  the  habitable  globe,  has  always  pro- 
^  vided  effectual  means  for  the  accomplishment  of  these  im- 
portant ends.  The  seeds  of  plants  are  either  scattered  in 
the  immediate  neighbourhood  of  the  parent,  and  take  root 
in  the  adjacent  soil,  or  are  carried  to  more  distant  situations 
by  the  wind  or  other  agents.  In  the  animal  kingdom,  the 
young  offspring  of  those  races  which  are  endowed  with  a 
wide  range  of  activity,  are  reared  on  the  spot  where  they 
were  produced,  either  by  the  fostering  care  of  the  parent, 
or  by  means  of  the  nourishment  with  which  they  are  sur- 
rounded in  the  egg,  and  there  remain  until  the  period  when, 
by  the  acquisition  or  extension  of  locomotive  powers,  they 
are  enabled,  in  their  turn,  to  go  in  quest  of  food.  But  in  the 
tribes  of  animals  at  present  under  our  consideration,  this  or- 
der is  reversed.  It  is  the  parent  that  is  chained  to  the  same 
spot  from  an  early  period  of  its  growth,  and  it  is  on  the  young 
that  the  active  powers  of  locomotion  have  been  conferred, 
apparently  for  the  sole  purpose  of  seeking  for  itself  a  proper 
habitation  at  some  distance  from  the  place  of  its  birth;  and 
when  once  it  has  made  this  selection,  it  there  fixes  itself  un- 
alterably for  the  remaining  term  of  its  existence."^ 

•  Phenomena,  which  appear  to  bear  some  analogy  with  these,  have  been 


SPONGES.  119 

The  parts  of  the  Spongia  panicea,  which  are  naturally 
transparent,  contain  at  certain  seasons  a  multitude  of  opaque 
yellow  spots,  visible  to  the  naked  eye,  and  which,  when  ex- 
^  amined  by  means  of  a  microscope,  are  found  to  consist  of 
groups  of  ova,  or  more  properly  gemmules,^  since  we  can- 
not discover  that  they  are  furnished  with  any  envelope. 
In  the  course  of  a  few  months  these  gemmules  enlarge  in 
size,  each  assuming  an  oval  or  pear-like  shape,  and  are  then 
seen  projecting  from  the  sides  of  the  internal  canals  of  the 
parent,  to  which  they  adhere  by  their  narrow  extremities. 
In  process  of  time  they  become  detached,  one  after  the  other, 
and  are  swept  along  by  the  currents  of  fluid,  which  are  ra- 
pidly passing  out  of  the  larger  orifices.  Fig.  55,  represents 
one  of  these  gemmules  detached  from  the  parent  sponge. 
When  thus  set  at  liberty,  they  do  not  sink  by  their  gravity 
to  the  bottom  of  the  water,  as  would  have  happened  had 
they  been  devoid  of  life;  but  they  continue  to  swim  by  their 
own  spontaneous  motions,  for  two  or  three  days  after  their 
separation  from  the  parent.  In  their  progression  through 
the  fluid  they  are  observed  always  to  carry  their  rounded 
broad  extremity  forwards.  On  examining  this  part  with  the 
microscope,  we  find  that  it  is  covered  with  short  fila- 
ments, or  cilia,  which  are  in  constant  and  rapid  vibration. 
These  cilia  are  spread  over  about  two-thirds  of  the  surface 
of  the  body,  leaving  the  narrower  portion,  which  has  a  whiter 
and  more  pellucid  appearance,  uncovered.  They  are  very 
minute  transparent  filaments,  broadest  at  their  base,  and  ta- 
pering to  invisible  points  at  their  extremities:  they  strike 

noticed  in  the  veg-etable  kingdom.  The  tribe  of  Zoocarpia,  produce  a  kind 
of  fruit,  which,  when  detached  from  the  parent,  appears  to  possess  powers 
of  spontaneous  motion,  until  the  period  of  its  taking  root,  and  growing  like 
a  vegetable  structure.  These  singular  productions,  which  seem,  in  their 
progressive  developments,  to  possess  alternately  the  characters  of  vegetables 
and  of  animals,  may  perhaps  be  regarded  as  connecting  links  between  the 
two  great  kingdoms  of  living  nature. 

f  Gemmule  is  a  term  derived  from  the  Latin  word  geinma,  a  bud:  and  its 
meaning,  as  apphed  to  zoophytes,  is  that  of  a  young  animal,  not  contained 
within  an  envelope,  or  egg. 


120  THE  MECHANICAL  FUNCTIONS. 

the  water  by  a  rapid  succession  of  inflections,  apparently 
made  without  any  regular  order,  but  conspiring  to  give  an 
impulse  in  a  particular  direction.  When  the  body  is  at- 
tached by  its  tail,  or  narrow  end,  to  some  fixed  object,  the 
motion  of  the  cilia  on  the  fore  part  of  the  body  determines 
a  current  of  fluid  to  pass  in  a  direction  backwards,  or  to- 
wards the  tail;  but  when  they  are  floating  in  the  water,  the 
same  action  propels  them  forwards  in  the  opposite  direction, 
that  is,  with  the  broad  ciliated  extremity  foremost.  They 
thus  advance,  without  appearing  to  have  any  definite  object, 
by  a  slow  gliding  motion,  totally  unlike  the  zig-zag  course 
of  animalcules  in  search  of  prey.  Yet  they  appear  to  have 
a  consciousness  of  impressions  made  on  them;  for  on  striking 
against  each  other,  or  meeting  any  obstacle,  they  retard  a 
little  the  motion  of  their  cilia,  wheel  for  a  few  seconds 
round  the  spot,  and  then,  renewing  the  vibrations,  proceed 
in  their  former  course. 

In  about  two  or  three  days  after  these  gemmules  have  quit- 
ted the  body  of  the  parent,  they  are  observed  to  fix  themselves 
on  the  sides  or  bottom  of  the  vessel  in  which  they  are  con- 
tained; and  some  of  them  are  found  spread  out,  like  a  thin  cir- 
cular membrane  on  the  surface  of  the  water.  In  the  former 
case,  they  adhere  firmly  by  their  narrow  extremity,  which  is 
seen  gradually  to  expand  itself  laterally,  so  as  to  form  a  broad 
base  of  attachment.  While  this  is  going  on,  the  cilia  are  still 
kept  in  rapid  motion  on  the  upper  part,  scattering  the  opaque 
particles  which  may  happen  to  be  in  the  fluid,  to  a  certain 
distance  around.  But  these  motions  soon  become  languid, 
and,  in  the  course  of  a  few  hours,  cease;  and  the  cilia,  being 
no  longer  wanted,  disappear.  The  gemmule  then  presents 
the  appearance  of  a  flattened  disk,  containing  granules,  like 
the  flesh  of  the  parent  sponge;  and  also  several  spiculae  in- 
terspersed through  the  central  part.  In  less  than  twenty- 
four  hours,  a  transparent  colourless  margin  has  extended 
round  the  whole  gemmule,  and  continues  to  surround  it 
during  its  future  growth.  The  spicula,  which  were  at  first 
small,  confined  to  the  central  part,  and  not  exceeding  twenty 
in  number,  now  become  much  larger  and  more  numerous; 


SPONGES.  121 

and  some  of  them  shoot  into  the  thin  homogeneous  mnrcin. 
It  is  a  rcmarkahle  circumstance  that  the  spicula  make  their 
appearance  completely  formed,  as  if  by  a  sudden  act  of 
crystallization,  and  never  afterwards  increase  their  dimen- 
sions. 

When  two  gemmules,  in  the  course  of  their  spreading  on 
the  surface  of  a  watch-glass,  come  into  contact  with  each 
other,  their  clear  margins  unite  without  the  least  interrup- 
tion; they  thicken  and  produce  spicula:  in  a  few  days  we 
can  detect  no  line  of  distinction  between  them,  and  they 
continue  to  grow  as  one  animal.  The  same  thing  happens, 
according  to  the  observation  of  Cavolini,  to  adult  sponges, 
which,  on  coming  into  mutual  contact,  grow  together  and 
form  an  inseparable  union.  In  this  species  of  animal  graft- 
ing we  again  find  an  analogy  between  the  constitution  of 
zoophytes  and  that  of  plants. 

In  the  course  of  a  few  weeks,  the  spicula  are  assembled 
in  groups,  similar  to  those  of  the  parent  sponge;  assuming 
circular  arrangements,  and  presenting  distinct  openings  at 
the  points  they  enclose.  The  young  animal  now  rapidly 
spreads  and  enlarges  in  every  direction,  becoming  more 
convex,  and  at  the  same  time  more  opaque,  and  more  com- 
pact in  its  texture;  and  before  it  has  attained  the  tenth  of  an 
inch  in  diameter,  it  presents,  through  the  microscope,  a  mi- 
niature representation  of  its  parent. 

Thus  has  a  power  of  spontaneous  motion  been  given  to 
what  may  be  regarded  as   the  embryo  condition  of  animals, 
which  are  afterwards  so  remarkable  for  their  inertness,  and 
for  the  privation  of  all  active  powers:  and  this  has  been  con- 
ferred evidently  for  the  purpose  of  their  being  widely  disse- 
minated over  the  globe.     Had  not  this  apparatus  of  moving 
cilia  been   provided  to  the  gemmules  of  such    species  as 
hang  vertically  from  the  roofs  of  caves,  they  would  have 
sunk  to  the  bottom  of  the  water  and  been  crushed  or  buried 
among  the  moving  sand,  instead  of  su])porting  themselves 
while  carried  to  a  distance  by  the  waves  and  tides  of  the 
ocean.     Many  species  which  abound  in  the  Red  Sea  and 

Indian  Ocean  have,  in  this  way,  been  gradually  transported. 
Vol.  I.  16 


122 


THE  MECHANICAL  FUNCTIONS. 


by  the  Gulf  stream,  from  the  shores  of  the  east  to  corre- 
sponding latitudes  of  the  new  world. 


§  3.  Polypifera, 

The  next  step  in  the  organic  series  introduces  us  to  the 
extensive  family  of  Polypifera.  The  transition  from  the 
structure  of  the  sponge  to  that  of  the  polypus  may  be  thus 
conceived.  Suppose  the  absorbing  orifices  of  the  former  to 
be  enlarged,  and  their  number  to  be  at  the  same  time  re- 
duced: and  let  these  orifices  be  drawn  out  into  tubes,  and 
provided  with  vibratory  cilia;  in  addition  to  which,  let 
there  be  placed  around  their  margin  a  circular  row  of  larger 
filaments,  extremely  flexible,  and  capable  of  twining  round 
any  object  that  comes  within  their  reach,  and  of  conveying 
it  to  the  central  orifice,  which  performs  the  office  of  a  mouth. 
Each  tube,  thus  furnished  with  a  circle  of  radiating  fila- 
ments, or  tentacula,  as  they  are  called,  is  denominated  a 
Polype.^'  The  animal  structure  thus  composed  has  received 
the  name  of  Lobularia  (Fig.  5Q,)  and  is  the  genus  among 


•  For  the  sake  of  greater  distinctness  I  shall  employ  the  term  polype  to 
denote  the  sing-le  tube  with  its  tentacula;  and  shall  designate  by  the  Latin 
term  polypus  the  entire  animal  mass  composed  of  an  aggregation  of  these 
polypes.  rolypiferUy  the  name  of  the  order,  expresses  animals  bearing  po- 
lypes. 


POLYPI.  123 

this  tribe  that  approaches  the  nearest  in  its  character  to  the 
sponge,  which  it  resembles  in  the  nature  of  its  internal  tex- 
ture. Each  of  the  polypes  with  which  its  surface  is  studded 
has  eight  serrated  tentacula.  Fig.  57  represents  one  of  these 
polypes  detached.  Polypes  may  thus  be  united  in  immense 
numbers  in  one  mass,  having  mutual  organic  connexion.  In 
other  cases  they  may  form  smaller  clusters,  or  be  even  to- 
tally unconnected.  Sometimes  the  detached  polypes  are 
still  disposed  to  assemble  in  groups,  as  is  the  case  with  the 
Zoanthus  of  Cuvier*  (Fig.  5S:)  at  other  times  they  are  al- 
together isolated,  as  in  the  Hydra  viridis  (Fig.  59.) 

Polypi  form  a  very  extensive  order  of  zoophytes,  abound- 
ing in  every  part  of  the  ocean,  but  growing  in  greatest  lux- 
uriance in  the  warmer  regions  of  the  globe.     Their  flesh 
exhibits  the  same  granular  appearance  as  that  of  the  spon«rc, 
but  it  is  generally  firmer,  and  often  intermingled  with  masses 
of  calcareous  matter.     The  tentacula,  which  may  be  com- 
pared to  arms,  vary  in  number  and  in  length  in  different 
species  of  polypi,  and  sometimes,  instead  of  a  single  row, 
each  of  the  mouths  has  two  or  more  series  of  tentacula  placed 
around  it.     They  are  formed  of  a  prolongation  of  the  soft 
substance  of  the  polypus,  and  are  sometimes  tubular;  and 
their  cavities  are  then  continuous  w^th  that  of  the  general 
internal  cavity  into  which  the  several  mouths  open.     Be- 
sides being  flexible  in  every  direction,  the  tentacula  are  also 
capable  of  being  lengthened  or  shortened  at  the  pleasure  of 
the  animal.     Their  elongation  is  produced  by  the  propulsion 
of  a  fluid  into  their  interior,  derived  from  the  general  cavity 
of  the  body;  and  their  retraction  is  effected  by  the  return  of 
the  same  fluid. 

The  whole  arrangement  of  the  tentacula  on  the  margin  of 
the  projecting  mouths  bears  a  striking  resemblance  to  a 
flower,  especially  to  those  which,  like  the  daisy,  or  china- 
aster,  have  the  corolla  composed  of  slender  radiating  petals. 
We  find,  indeed,  that  as  the  organs  of  zoophytes  become 
more  developed,  the  alfmities  which  these  lower  departments 

•  The  Hydra  sociata  of  Gmclln;  the  Acl'mia  sociata  of  Kills. 


124  THE  MECHANICAL  FUNCTIONS. 

of  the  animal  kingdom  retain  with  plants,  are  more  marked 
and  more  predominant.  In  the  construction  of  zoophytes, 
nature  seems  still  to  keep  in  view  the  models  of  vegetable 
forms,  the  characters  of  which,  while  effecting  the  transition 
from  one  kingdom  to  the  other,  she  continues  to  impress  on 
her  productions.  Zoophytes,  both  in  their  outward  form, 
and  in  the  disposition  of  their  internal  organs,  preserve  the 
symmetrical  arrangement  round  a  common  centre  so  gene- 
rally exhibited  in  plants,  and  especially  in  flowers,  and  in 
the  verticillated  leaves  and  branches.*  Hence,  the  radiated 
or  star-like  forms  which  predominate  in  most  of  the  animals 
cbmposing  this  class:  and,  hence,  they  have  obtained  the  ti- 
tle of  Radiata,  by  which  Cuvier  has  designated  them. 

Like  the  animals  of  the  sponge  tribe.  Polypi  are  for  the 
most  part  attached  to  some  inorganic  shell  or  base,  which 
may  be  either  of  a  horny  or  calcareous  nature.  The  form 
of  this  shell  admits  of  almost  infinite  variety.  In  some  it 
constitutes  the  external  surface  of  the  animal,  and  encloses 
the  flesh  in  a  general  sheath,  leaving  only  openings  at  the 
extremities  of  the  tubes  for  the  expansion  of  each  set  of 
tentacula  surrounding  the  respective  mouths.  Sometimes 
these  tubes  are  placed  parallel  to  each  other,  like  the  pipes 
of  an  organ,  with  transverse  partitions  at  regular  intervals: 
such  is  the  structure  of  the  Tubipora  miisica,  as  shown  in 

Fig.  61.  In  Fig.  62,  a  portion  of  the 
tubes  is  seen  highly  magnified,  and  laid 
open,  to  show  the  polypes  in  their  inte- 
rior. At  other  times  the  tubes  are  joined 
together  endwise,  like  the  branches  of  a 
tree,  leaving  lateral  apertures  for  the  pro- 
trusion of  the  tentacula  of  each  separate 
polype:  this  is  the  case  in  the  Seriularia. 
(Fig.  60.) 

In  some  species  the  horny  base  is  fashioned  into  a  num- 
ber of  cells,  each  of  which  serves  for  the  protection  of  its 
respective  polype.     These  cells  are  generally  placed  at  the 

*  See  page  7Q. 


POLYPI. 


125 


extremity  of  the  branches,  presenting  the  greatest  similitucle 
to  flowers.     The  Flu^lra  (Fig.  63)  is  composed  of  minute 


61 


62 


63 


64 


and  almost  microscopic  cells,  spread  over  a  flat  membra- 
neous substance,  resembling,  in  the  flexibility  of  its  texture, 
and  its  mode  of  subdivision,  the  leaves  of  plants.  These 
cells  are  arranged  in  rows,  with  great  regularity,  like  those 
of  a  honey-comb,  as  is  seen  in  the  magnified  view  of  them, 
Fig.  64. 

In  other  tribes  the  inorganic  base  of  support  is  internal, 
constituting  a  kind  of  skeleton  or  axis;  the  polypous  mouths 
being  spread  at  intervals  over  the  surface  of  the  fleshy  lay- 
er which  covers  this  skeleton.  This  is  the  case  with  the 
Gorgonia,  jSntipathes,  and  the  Coral,  which  exhibit  still 
closer  resemblances  to  the  branched  forms  of  vegetable  stems. 
The  flesh  contains  granules  of  calcareous  matter,  which,  in  the 
dried  specimens,  adhere  to  the  surface  of  the  stems.  Fig. 
G5  is  a  branch  of  the  Corallium  rubum,  of  w^hich  Fig.  ^G 


is  a  magnified  portion,  showing  the  appearance  of  the  poly- 
pes in  their  expanded  and  contracted  states.     The  way  in 
which  the  polypes  are  embedded  in  the  flesh  is  seen  in  Fig. 
67,  which  represents  a  section  of  the  Gorgonia  Briareus, 
In  many  cases  the  polypes  arc  lodged  in  cup-like  depres- 


126  THE  MECHANICAL  FUNCTIONS.  * 

sions  in  the  surface  of  the  calcareous  axis,  which  affords  them 
some  degree  of  protection.  In  Madrepores  these  depres- 
sions are  crossed  by  radiating  plates,  adapted  to  the  form 
and  number  of  the  tentacula.  In  Millej)ores  the  cells  are 
closer  and  more  minute,  and  exhibit  none  of  these  star-like 
radiations.  In  some  species  the  plates  have  more  of  a  pa- 
rallel arrangement;  and  in  others  they  form  a  net-work. 

The  material  of  which  this  axis,  to  which  the  polypes  are 
attached,  is  composed,  is  of  various  kinds.  Sometimes  it  is 
horny,  flexible,  and  elastic,  corresponding  in  its  nature  to 
animal  membrane:  at  other  times  it  is  hard  and  calcareous, 
being  composed  principally  of  carbonate  of  lime,  with  a 
small  quantity  of  the  phosphate;  the  proportion  of  this  latter 
ingredient  varying  in  different  species.  In  all  cases  the 
particles  of  calcareous  matter  are  united  together  by  some 
portion  of  animal  substance  which  may  be  obtained  by  dis- 
solving out  the  former  by  an  acid.  We  always  find  the  ma- 
terials arranged  in  concentric  layers,  indicating  that  their 
deposition  has  been  successive;  and  the  surface  is  marked 
by  longitudinal  lines,  corresponding  to  the  figure  of  the  ani- 
mal covering  of  flesh.  Sometimes  the  stem  consists  of  horny 
and  calcareous  parts  disposed  alternately,  composing  a  jointed 
structure,  which  some  have  fancied  might  be  considered  as 
making  an  approach  to  an  articulated  skeleton;  for  it  is  ca- 
pable of  considerable  flexion,  and  readily  yields  to  the  im- 
pulse of  the  waves,  without  the  risk  of  being  broken.  This 
is  the  case  with  the  his  hipj)uris,  commonly  known  by  the 
name  oi  jointed  coral.  (Fig.  68.)  There  is,  in  short,  hardly 
any  possible  combination  of  these  parts  which  does  not  oc- 
casionally occur  amidst  the  infinite  diversities  of  condition 
displayed  in  this  department  of  the  animal  creation. 

These  structures  are  generally  attached  to  submarine  rocks 
by  an  expansion  of  the  base  into  a  kind  of  foot,  or  root, 
which  has  a  strong  power  of  adhesion.  In  this  respect, 
therefore,  as  in  so  many  others,  these  animals  preserve  an 
analogy  with  plants. 

It  has  been  ascertained  that,  in  a  great  number  of  instances, 
these  fixed  zoophytes  are  multiplied,  like  the  sponge,  by  the 


POLYPI.  127 

detachment  of  gemmulcs,  or  imperfectly  formed  portions  of 
their  soft  substance.     These  gcmmules  require  to  undergo 
the  same  kind  of  metamorphosis  in  order  to  bring  them  to 
their  perfect  state;  and  when  newly  detached  from  the  pa- 
rent, they  exhibit  the  same  singular  spontaneous  motions, 
buoying  themselves  in  the  water,  and  swimming  in  various 
directions,  by  the  rapid  vibrations  of  their  cHla,  till  they 
find  a  place  favourable  to  their  growth.     On  becoming  fixed, 
they  spread  out  to  form  a  base  for  the  future  superstructure; 
and,  after  the  foundation  has  thus  been  laid,  they  proceed  in 
their  upward  growth,  depositing  a  calcareous  or  horny  axis 
in  successive  layers,  until  it  has  acquired  the  requisite  thick- 
ness; and  they  then  gradually  assume  the  forms  character- 
istic of  the  particular  species  to  which  they  belong.     The 
materials  thus  deposited  are  permanent  structures,  not  ca- 
pable of  modification  or  removal,  and  not  possessing  any 
vital  properties;   for  these  properties  belong  exclusively  to 
the  animated  flesh  with  which  these  structures  are  associated. 
The  polypes  themselves  are  not  developed  till  after  the  for- 
mation of  the  root  and  stem;  their  growth  being  in  this  re- 
spect analogous  to  that  of  the  leaves  and  flowers  of  a  plant. 
The  gemmules  of  the  Flustra  carbasea  may  be  selected 
in  illustration  of  this  phenomena.     These  have   been   ob- 
served by  Dr.  Grant,*  to  swim  about  in  the  water  as  soon  as 
they  have  escaped  from  the  cells  of  the  parent;  each  moving 
with  its  narrow  end  foremost,   while    the  opposite  broad 
end,  which  is  covered  with  cilia,  expands  into  a  flat  circular 
zone.     These  gemmules  are  very  irritable,  and  are  frequent- 
ly seen  to  contract  the  circular  margin  of  their  broad  ex- 
tremity; and,  while  swimming,  to  stop  suddenly  in  their 
course.     They  swim  with  a  gentle  gliding  motion:  at  other 
times  they  appear  stationary,  all  the  while  revolving  rapidly 
round  their  longer  axis,  with  their  broad  end  uppermost:  they 
often  bound  forwards,  either  in  straight  lines,  or  describing 
circles,  with  no  other  apparent  object  than  to  keep  them- 
selves afloat,  until  they  shall  arrive  at  a  favourable  spot  for 

*  Edinburgh  Phllosoplncal  Journal,  XYII.  107  and  'oo7. 


128  THE  MECHANICAL  FUNCTIONS. 

fixing  their  permanent  abode,  and  proceeding  in  their  far- 
ther development.  The  tune  of  their  remaining  in  this  free 
and  moving  state  varies  according  to  circumstances,  from  a 
few  hours  to  about  three  days.  When  about  to  fix,  the 
slightest  agitation  of  the  water  causes  them  to  desist,  and  to 
recommence  their  gliding  motions,  which  they  continue  for 
some  time  longer.  If,  when  any  of  these  gemmules  has  be- 
gun to  fix,  it  be  again  disturbed,  and  separated  from  the 
surface  to  which  it  had  become  attached,  it  generally  re- 
mains free,  and  perishes.  During  the  process  of  fixing,  it 
exhibits  no  peculiar  appearance  or  change  of  form;  it  simply 
lies  on  its  side:  and  the  cilia  continue  to  vibrate  over  the 
whole  surface,  producing  a  constant  current  in  the  water, 
apparently  for  the  purpose  of  cleaning  the  space  immediate- 
ly surrounding  the  gemmule.  It  remains  for  three  days  in 
this  attitude,  without  undergoing  any  perceptible  change 
of  form,  and  without  relaxing  the  vibrations  of  its  cilia.  At 
the  end  of  this  time,  the  cilia  cease  to  move,  and  shortly  af- 
ter disappear:  then  the  gemmule  begins  to  swell,  the  sur- 
rounding margin  becomes  more  transparent,  and  the  whole 
gradually  assumes  the  form  of  a  cell,  surrounded  by  a  deli- 
cate white  opaque  line,  which  is  the  rudiment  of  the  calca- 
reous wall  of  the  future  cell.  Towards  the  base  of  this  ru- 
dimental  cell,  the  gelatinous  substance  in  the  interior  may 
be  perceived  to  become  more  consistent  and  opaque  at  a 
particular  point;  from  this  dull  spot  within  the  cell,  short 
straight  tentacula  begin  to  bud,  extending  upwards  in  the  di- 
rection of  the  future  aperture.  The  gelatinous  spot,  from 
which  the  tentacula  originated,  assumes  the  vermiform  ap- 
pearance of  the  body  of  a  polype;  and  we  may  distinctly 
perceive  the  bundles  of  fibres  which  connect  its  head  with 
the  base  of  the  cell.  The  structure  of  the  polype  is  per- 
fected by  the  addition  of  a  closed  capsule;  and  when  it  is 
first  detected  protruding  from  the  cell,  it  possesses  all  the 
parts  of  an  adult  polype,  and  vibrates  the  cilia  of  its  tenta- 
cula vvith  as  much  regularity  and  velocity  as  at  any  future 
period.     Before  the  polype  is  capable  of  protruding  from 


POLYPI. 


129 


the  aperture  of  the  first  cell,  the  upper  part  of  the  cell  has 
already  extended  outwards  to  form  the  rudiment  of  a  second: 
and  so  on,  in  succession,  till  the  whole  structure  is  completed. 
The  tentacula  of  polypi  are  exquisitely  sensible,  and  are 
frequently  seen,  either  singly  or  altogether,  bending  their 
extremities  towards  the  mouth,  when  any  minute  floating 
body  comes  in  contact  with  them.  When  a  polype  is  ex- 
panded, a  constant  current  of  water  is  observed  to  take 
place,  directed  towards  the  mouth.  These  currents  are 
never  produced  by  the  motions  of  the  tentacula  themselves; 
but  are  invariably  the  effects  of  the  rapid  vibrations  of  the 

cilia  placed  on  the  tentacula.  In  the 
polypes  of  the  Flustra  carbasea, 
(Fig.  69,)  the  tentacula  have  each  a 
single  row  of  cilia,  extending  along 
both  the  lateral  margins,  from  their 
base  to  their  termination.*  Each 
polype  has  usually  twenty-two  ten- 
tacula; and  there  are  about  fifty  cilia 
on  each  side  of  a  tentaculum,  makins: 
2200  cilia  on  each  polype.  As  there 
are  above  1800  cells  in  each  square 
inch  of  surface,  and  the  branches  of  an  ordinary  specimen 
present  about  ten  square  inches  of  surface,  we  may  estimate 
that  an  ordinary  specimen  of  this  zoophyte  presents  more 
than  18,000  polypes,  396,000  tentacula,  and  39,600,000  ci- 
lia. But  other  species  certainly  contain  more  than  ten  times 
these  numbers.t 

The  vibrations  of  these  cilia  are  far  too  rapid  to  be  fol- 
lowed by  the  quickest  eye,  even  when  assisted  by  the  most 
powerful  microscope,  and  can  be  detected  only  at  the  times 
when  they  have  become  comparatively  languid,  by  the  di- 
minished vigour  of  the  animal:  their  motions  may  then  be 

•  A  portion  of  one  of  these  tentacula  is  represented,  highly  mag-nified,  in 
Fig-.  70.  The  lower  figure,  (g,)  is  the  delineation  of  one  of  the  gemmulcs 
of  the  same  polypus,  also  greatly  magnified. 

f  Dr.  Grant  has  calculated  that  there  are  about  400,000,000  cilia  on  a  sin- 
gle Flustra  foliacea.  Transactions  of  the  Zoological  Society  of  London, 
Vol.  i.  p.  11. 

Vol.  I.  17 


130  THE  MECHANICAL  FUNCTIONS. 

seen,  ascending  on  one  side  of  the  tentaculum  and  descend- 
ing on  the  other.  (Fig.  70.)  All  the  cilia  appear  to  com- 
mence and  to  cease  their  motions  at  the  same  moment.  The 
constancy  with  which  they  continue  would  seem  to  exclude 
the  possibility  of  their  being  the  result  of  volition;  and  they 
are,  therefore,  more  probably  determined  by  some  unknown 
ph)'sical  cause,  dependent,  however,  on  the  life  of  the  ani- 
mal. But  so  retentive  are  they  of  the  power  of  motion-, 
whatever  may  be  its  cause,  that  if  any  one  of  the  tentacula 
be  cut  off,  its  cilia  will  continue  to  vibrate,  and  will  propel 
it  forward  in  the  fluid  for  a  considerable  time,  as  if  it  had 
become  itself  an  individual  animal. 

A  question  arises  with  regard  to  the  constitution  of  these 
zoophytes,  similar  to  that  which  has  been  proposed  with  re- 
gard to  trees,  namely,  what  limits  should  be  assigned  to 
their  individuality?  Is  the  whole  mass,  which  appears  ta 
grow  from  one  root,  and  which  consists  of  multitudes  of 
branches,  proceeding  from  a  common  stem,  to  be  considered 
as  one  individual  animal,  or  is  it  an  assemblage  or  aggrega- 
tion of  smaller  individuals:  each  individual  being  characte- 
rized by  having  a  single  mouth,  with  its  accom.panying  ten- 
tacula, and  yet  the  whole  being  animated  by  a  common 
principle  of  life  and  growth?  The  greater  number  of  natu- 
ralists have  adopted  this  latter  view,  regarding  each  portion, 
so  provided  with  a  distinct  circle  of  tentacula,  as  a  separate 
animal,  associated  with  its  neighbours  in  the  construction  of 
a  common  habitation,  and  contributing  its  quota  to  the  ge- 
neral nourishment  of  this  animal  republic.  As  the  deter- 
mination of  this  question  involves  tlie  consideration  of  the 
function  of  nutrition,  I  shall  postpone  its  farther  discussion 
to  a  future  part  of  this  treatise.  As  far,  indeed,  as  regards 
the  mechanical  condition  of  animals  which  are  so  complete- 
ly stationary,  it  matters  little,  whether  the  whole  mass  be 
regarded  as  one  individual  animal,  or  as  an  aggregate  of  dis- 
tinct individuals.  But  the  question  becomes  of  some  impor- 
tance when  applied  to  detached  zoophytes,  such  as  Fenna- 
iiila,  which  are  formed  of  a  multitude  of  polypes  connected 
with  a  common  stem,  but  w^hich  float  at  liberty  in  the  sea. 


PENNATULA. 


131 


The  Pennatula,  (Fig.  71,)  has  been  termed  the  sea  pen, 

from  tlie  circumstance  of  its  calcare- 
ous axis,  or  stem,  having  a  double 
set   of  branches,   extending   in    the 
same  plane  from   both  the  sides,  like 
the  vane  of  a  quill,  and  of  its  series 
of  polypes  being  set  along  one  edge 
of  each    branch,   like  the  filaments 
which  arise  from   the  fibres  of  the 
feather.     Some  of  these  polypes  are 
seen  magnified  in  Fig.  72.     Immense 
numbers  of  these  curious  animals  are 
met  with  in  different  parts  of  the  ocean.     If  they  possessed 
in  any  degree  the  power  of  locomotion,  which  many  natura- 
lists have  ascribed  to  them,  we  should  be  able  to  ascertain 
wdiether  all  their  movements  are  conducted  by  a  common 
volition,  or  whether  they  are  performed  independently  of 
one  another.     It  has  often,  indeed,  been  asserted,  that  pen- 
natulse  swim  through  the  water  by  their  own  spontaneous 
movements,  consisting  either  in  the  waving  up  and  down  of 
the  lateral  branches,  or  in  the  simultaneous  impulses  of  the 
tentacula  of  all  the  polypes.     Cuvier  even  represents  the 
polypes  of  the  pennatula  as  having  the  power  of  keeping 
time,  while  they  are  waving  the  mass  through  the  water,  as 
if  they  were  all  actuated  by  a  single  undivided  volition. 
But  Dr.  Grant,  who  has  watched  the  motions  of  these  ani- 
mals with  great  care,  is  led  by  his  observations  to  the  con- 
clusion that  pennatulae  are  not  in  reality  possessed  of  an}'- 
such  locomotive  faculty;  but  that  they  are  carried  to  and  fro 
in  the  ocean,  like  the  gulf  weed,  without  the  slightest  vo- 
luntary power  of  directing  their  course.     Whatever  may  be 
the  result  of  the  combined  movements  of  the  tentacula,  the 
arms  are  certainly  incapable  of  those  inflections  which  have 
been  supposed  to  supply  the  means  of  progressiv^e  motion. 

It  is  only  when  the  contractile  flesh  of  the  polypus  is  re- 
leased from  the  restraint  which  the  solid  axis  imposes  upon 
its  movements,  that  the  animal  becomes  capable  of  any  dis- 
tinct power  of  locomotion.     Such  is  the  condition  of  the 


132  THE  MECHANICAL  FUNCTIONS. 

animals  belonging  to  the  genus  Hydra,  of  which  the  Hydra 
viridis,  or  fresh  water  polype,  (Fig.  59,  p.  122)  may  be  taken 
as  the  type.  This  singular  animal  presents  us  with  perhaps 
the  simplest  kind  of  structure  that  exists  in  the  animal  king- 
dom. It  would  almost  seem  as  if  Nature  had  formed  it  with 
the  design  of  exhibiting  to  us  the  resources  of  vitality  in 
carrying  on  the  functions  of  animal  life  without  the  aid  of 
the  complicated  apparatus  which  she  has  bestowed  upon  the 
higher  orders  of  the  creation.  The  Hydra  consists  merely  of 
a  fleshy  tube,  open  at  both  ends,  one  of  which,  being  more 
dilated,  may  be  regarded  as  the  head,  and  has  for  a  mouth 
the  aperture  of  the  tube,  which  is  furnished  at  its  margin 
with  a  single  row  of  tentacula.  It  thus  corresponds  to  the 
general  definition  of  a  polypus,  and  exemplifies  its  most 
simple  form. 

The  whole  body  may,  on  the  one  hand,  be  considerably 
elongated,  and  on  the  other,  so  much  retracted,  as  to  appear 
a  mere  globule;  and  these  movements  are  the  efiect  of  a  vo- 
luntary power  in  the  animal  directed  to  specific  ends.  The 
number  of  tentacula  varies  from  six  to  twelve;  they  are  slen- 
der tubular  filaments,  capable  of  being  extended  to  a  great 
length,  and  of  being  bent  in  all  directions.  In  this  way, 
they  can  quickly  surround  and  grasp  any  small  object  which 
they  may  happen  to  touch;  and  whenever  irritated  they  in- 
stantly retract,  so  as  hardly  to  be  visible  without  the  aid  of 
a  magnifier.  Each  tentaculum  may  be  moved  independent- 
ly of  the  rest,  at  the  pleasure  of  the  animal.  The  remainder 
of  the  body  tapers  gradually  from  the  head  to  the  other  ex- 
tremity, becoming  very  slender,  and  having  at  its  termina- 
tion a  flat  surface,  which  has  been  termed  the  foot;  for  al- 
though every  portion  of  the  surface  has  the  power  of  ad- 
hering to  the  bodies  to  which  it  is  applied,  it  is  principally 
by  this  extremity  that  the  animal  chooses  to  attach  itself  to 
the  sides  or  bottom  of  the  vessel  in  which  it  is  kept.  No 
trace  of  the  existence  of  cilia  can  be  met  with  on  any  part 
of  the  surface  of  these  animals. 

It  is  to  Mr.  Trembley  of  Geneva  that  we  are  indebted  for 
the  discovery  of  this  singular  animal,  the  examination  of 


HYDRA.  1 33 

which  has  contributed  to  throw  great  light  on  the  natural 
history  of  polypiferous  animals.^  While  observing  some 
a(juatic  plants,  which  he  had  collected  and  put  into  water^ 
his  attention  was  called  to  the  aj)pearance  of  filaments  ad- 
hering to  them,  wiiich  he  had  first  conceived  to  be  parasitic 
vegetables:  but  farther  observation  convinced  him  that  they 
were  endowed  with  powers  of  spontaneous  motion,  and  that 
they  preyed  upon  small  insects:  and  he,  therefore,  could  no 
longer  doubt  their  animal  nature.  He  found  that  they  al- 
ways placed  themselves  on  the  side  of  the  glass  next  to  the 
light;  and  by  watching  their  changes  of  position,  he  disco- 
vered the  mode  in  which  they  effect  their  progressive  mo- 
tions. If  the  hydra  be  standing  in  the  erect  position,  its 
foot  being  applied  to  the  bottom  of  the  glass  (Fig.  73, ^  it 
slowly  bends  the  body  in  the  direction  in  which  it  intends 
to  advance  till  its  head  touches  the  vessel,  as  shown  in  Fig. 
74.  It  then  adheres  to  the  surface  by  the  mouth,  or  by  one 
or  two  of  its  tentacula,  and,  detaching  the  foot,  bends  the 
body  into  a  curve,  at  the  same  time  slightly  retracting  it, 
so  that  the  foot  is  brought  near  the  head  (Fig.  75.)  The 
foot  is  then  again  fixed,  preparatory  to  a  new  step,  which  it 
takes  by  detaching  the  head  and  projecting  it  forwards  as 
before  (Fig.  76.) 


75 


76 


The  progress  made  by  these  successive  efforts  is  but  slow: 
for  the  hydra  often  pauses  in  the  midst  of  a  step,  as  if  de- 
liberating whether  it  should  proceed:  so  that  the  traversing 
a  distance  of  seven  or  eight  inches  is  to  these  animals  a  very 
good  day's  journey,  even  in  summer.  But  a  mode  of  tra- 
velling rather  more  expeditious  than  this  is  occasionally  re- 
sorted to.     It  consists  of  a  succession  of  somersets:  the  hy- 

*  Memolres  pour  servir  a  rHistoIro  d'lin  genre  de  Polypes  d'eau  douce, 
a  bras  en  forme  dc  cornes.     Par  A.  Tremblev,  1744. 


134  THE  MECHANICAL  FUNCTIONS. 

dra,  while  adhering  firmly  by  the  mouth,  detaches  its  foot, 
and,  making  it  describe  a  semicircle,  throws  it  over  its  head, 
and  places  it  foremost  in  the  line  of  progression.  Having 
attained  this  situation,  the  foot  is  then -fixed,  and  a  similar 
semi-revolution  is  performed  by  the  head,  the  body  conti- 
nuing all  the  while  elongated. 

By  these  and  other  manoeuvres  these  animals  contrive  to 
walk  with  equal  facility  in  any  direction,  either  on  the  bot- 
tom or  sides  of  the  vessel,  or  along  the  stems  of  aquatic 
plants,  to  which  they  are  most  frequently  found  attached. 
The  position  in  which  they  appear  to  take  most  delight,  is 
that  of  remaining  suspended  from  the  surface  of  the  water 
by  means  of  the  foot  alone:  and  this  they  effect  in  the  fol- 
io win  o-  manner.  When  the  flat  surface  of  the  foot  is  ex- 
posed  for  a  short  time  to  the  air,  above  the  surface  of  the 
water,  it  becomes  dry,  and  in  this  state  exerts  a  repulsive 
action  on  the  liquid:  so  that  when  dragged  below  the  level 
of  the  surface  by  the  weight  of  the  body  it  still  remains  un- 
covered, and  occupies  the  bottom  of  a  cup-shaped  hollow 
in  the  fluid,  thereby  receiving  a  degree  of  buoyancy  suffi- 
cient to  suspend  it  at  the  surface.  The  principle  is  the  same 
as  that  by  which  a  dry  needle  is  supported  on  water  in  the 
boat-like  hollow  which  is  formed  by  the  cohesive  force  of 
the  liquid,  if  care  be  taken  to  lay  the  needle  down  very  gen- 
tly on  the  surface.  If,  while  the  hydra  is  floating  in  this 
manner,  suspended  by  the  extremity  of  the  foot,  a  drop  of 
water  be  made  to  fall  upon  that  part,  so  as  to  wet  it,  this  hy- 
drostatic power  will  be  destroyed,  and  the  animal  will  im- 
mediately sink  to  the  bottom. 

While  in  this  state  of  suspension  from  the  surface,  the 
hydra  is  capable  of  performing  several  curious  evolutions, 
and  with  the  assistance  of  the  tentacula,  by  which  it  lays 
hold  of  objects  within  its  reach,  is  able  to  cross  over  from 
one  side  of  the  vessel  to  the  other.  It  does  not  appear  that 
these  animals  ever  employ  the  tentacula  as  instruments  for 
swimming;  but  they  frequently  use  them  as  cables,  or  an- 
chors, to  enable  them  to  retain  their  positions  in  security, 
however  violently  the  water  may  be  agitated.     Great  use  is 


HYDRA.  135 

also  made  of  the  tentacula  as  organs  of  prehension  for  seizing 
and  detaining  their  living  prey,  and  for  conveying  it  to  the 
mouth,  where  it  is  quickly  swallowed.  On  the  other  hand, 
vs^hen  alarmed,  or  exposed  to  irritation,  the  hydra  suddenly 
shrinks,  by  the  gradual  contraction  of  all  the  tentacula,  and 
of  the  body  also,  into  a  small  globule,  which  might  easily 
escape  notice,  unless  its  previous  situation  were  accurately 
observed. 

It  might  be  asked  by  what  power  is  this  animal,  occupying 
so  low  a  place  in  the  scale  of  organization,  enabled  to  per- 
form these  actions?  To  this  question,  however,  no  satisfac- 
tory answer  has  yet  been  given.  The  substance  of  the  hy- 
dra, when  examined  by  the  microscope,  appears  to  be  nearly 
homogeneous,  except  that  a  number  of  grains  are  intermixed 
with  the  pulpy  and  gelatinous  matter  composing  the  princi- 
pal bulk  of  the  body.  These  grains,  when  pressed  out  of 
the  flesh  into  water,  are  scattered  indiscriminately;  and  ap- 
pear to  have  been  united  in  the  living  animal,  by  means  of 
this  glutinous  material. 

No  perceptible  fibres,  either  muscular,  or  of  any  other 
kind,  can  be  detected  in  the  flesh  of  the  polypus:  nor  is 
there  the  least  indication  of  the  formation  of  transverse 
rings,  similar  to  those  which  exist  in  worms,  and  which,  in 
these  latter  animals,  contribute  to  progressive  motion.  Every 
portion  of  the  substance  of  the  body  is  equally  irritable  and 
contractile,  and  its  movements  appear  to  be  governed  by 
some  voluntary  power  belonging  to  the  animal,  and  direct- 
ed to  the  attainment  of  certain  ends.  The  softness  and  pli- 
ancy which  it  possesses  allow  of  its  being  closely  fitted  to 
all  the  inequalities  of  the  surface  of  the  bodies  to  which  it 
is  applied;  and  perhaps  this  cause  alone  occasions  it  to  ad- 
here with  great  force  to  these  bodies,  without  the  aid  of  any 
glutinous  fluid.  A  conjecture,  which  has  much  appearance 
of  probability,  has  been  ofi'ered,  that  this  power  of  adhesion 
is  derived  from  the  presence  of  a  great  numi)cr  of  exceed- 
ingly minute  disks,  interlj^ersed  over  every  part  of  the  sur- 
face, constituting  so  many  suckers,  and  resembling,  though 
on  a  very  diminutive  scale,  the  sucking  apparatus  on  the 
arms  of  the  cuttle-fish. 


136  THE  MECHANICAL  FUNCTIONS. 

The  Zoantlius  (Fig.  58)  belongs  to  a  tribe  of  larger  poly- 
pi, which  are  generally  met  with  assembled  in  clusters;  on 
which  account  it  is  termed  by  Ellis  the  Actinia  sociata,  or 
clustering  animal  flower.  It  consists  of  a  globular  body, 
leaving  a  mouth  surrounded  by  one  or  two  rows  of  tentacu- 
ja;  and  connected  below  with  a  firm  and  fleshy  tube,  which 
adheres  strongly  to  the  rocks  at  the  bottom  of  the  sea,  so 
that  it  remains  permanently  fixed  in  the  same  place. 

The  genus  Vorticella  is  formed  by  a  small  tribe  of  ani- 
mals, which,  although  they  have  been  usually  included  un- 
der the  present  order,  differ  from  polypi  in  having  no  tenta- 
cula,  but  only  cilia,  surrounding  the  margin  of  a  bell-shaped 
body,  which  is  mounted  upon  a  long  and  slender  foot-stalk 
(Fig.  77.*)  Currents  are,  as  usual,  excited  by  the  vibra- 
tions of  the  cilia;  which  in  the  simpler  species,  such  as  the 

Vorticella  cyathina,  here  delineated, 
are  the  efficient  instruments  of  progres- 
sive motion.  When  attached  by  its  foot- 
stalk, the  vorticella  advances  in  search  of 
food,  by  the  extension  of  the  foot-stalk 
into  a  straight  line;  but  quickly  retreats 
from  danger,  by  suddenly  throwing  it 
into  spiral  folds.  Many  of  the  species  of 
vorticellae  are  so  exceedingly  diminutive 
as  to  be  imperceptible  without  the  aid  of 
the  microscope.  They  conduct  us,  therefore,  by  a  natural 
gradation,  to  the  next  order  we  have  to  notice,  and  which 
is  composed  wholly  of  microscopic  animals. 

§  4.  Infusorna, 

The  Infusory  animalcules,  or  Infusoria,  were  so  named 
by  Muller,  a  Danish  naturalist,  from  the  circumstance  of 
their  swarming  in  all  infusions  of  vegetable  or  animal  sub- 
stances that  have  been  kept  for  a  sufficient  time.  They  are, 
in  general,  far  too  minute  to  be  perceptible  to  the  naked  eye: 
it  is  to  the  microscope  alone,  therefore,  that  we  owe  our 

*  They  also  dlffer'from  polypi  in  having  a  distinct  intestinal  canal,  with 
numerous  stomachs. 


INFUSORIA.  137 

knowledge  of  tlieir  existence,  and  of  the  curious  plicnomcna 
they  present:  yet  even  the  best  instruments  afford  us  but 
little  insight  into  their  real  organization  and  physical  condi- 
tions. On  this  account  it  is  extremely  diflicult  to  assi^rQ 
their  true  place  in  the  scale  of  animals.  By  most  systema- 
tic writers  they  have  been  regarded  as  occupying  the  very 
lowest  rank  in  the  series,  and  as  exemplifying  the  simplest 
of  all  possible  conditions  to  which  animal  life  can  ])e  re- 
duced. Monads,  which  are  the  smallest  of  visible  animal- 
cules have  been  spoken  of  as  constituting  *Uhe  ultimate 
term  of  animality;"  and  some  writers  have  even  expressed 
doubts  whether  they  really  belong  to  the  animal  kingdom, 
and  whether  they  should  not  rather  be  considered  as  the  ele- 
mentary molecules  of  organic  beings,  separated  from  each 
other  by  the  effects  of  chemical  decomposition,  and  retain- 
ing the  power  of  spontaneous,  but  irregular  and  indetermi- 
nate motion.  It  was  conceived  that  all  material  particles 
belong  to  the  one  or  the  other  of  two  classes;  the  first, 
wholly  inert  and  insusceptible  of  being  organized;  the  se- 
cond, endowed  with  a  principle  of  organic  aptitude,  or  ca- 
pability of  uniting  into  living  masses,  and  constituting,  there- 
fore, the  essential  elements  of  all  organization.  According 
■  to  this  view,  all  vegetables  or  animals  in  existence  would  be 
mere  aggregations  of  infusory  animalcules,  which  gradually 
accumulate  by  continual  additions  to  their  numbers,  de- 
rived from  organic  matter  in  the  food:  so  that  the  body  of 
man  himself  would  be  nothing  more  than  a  vast  consreca- 
tion  of  monads! 

This  bold  and  fanciful  hypothesis,  devised  by  Buffon, 
and  recommended  by  its  seductive  appearance  of  simplicity, 
as  well  as  by  the  glowing  style  and  brilliant  imagination  of 
its  author,  has  had  many  zealous  partisans.  The  new  world, 
which  was  disclosed  to  the  wondering  eyes  of  naturalists  by 
the  microscope,  abounding  in  objects  and  in  phenomena  of 
which  no  conception  could  have  been  formed  previously  to 
the  invention  of  that  instrument,  was  peculiarly  calculated 
to  excite  curiosity,  and  to  inspire  the  hope  of  its  revealing 
the  secret  of  the  living  principle  in  the  arrangement  of  the 

Vol.  I.  IS  .  ■ 


138  THE  MECHANICAL  FUNCTIONS. 

atoms  of  organic  bodies.  During  the  greater  part  of  the  last 
century,  infusory  animalcules  were  the  subject  of  frequent 
and  laborious  microscopical  research,  and  gave  rise  to  end- 
less conjecture  and  speculation  as  to  their  origin,  their  vi- 
tality, and  their  functions  in  the  economy  of  nature.  Not- 
withstanding their  minuteness,  considerable  differences  of 
organization  were  perceived  to  exist  among  them:  but  many 
naturalists  still  clung  to  the  idea  that  monads,  the  most  di- 
minutive of  the  tribe,  and  whose  very  presence  can  be  de- 
tected only  by  the  application  of  the  highest  magnifying 
powers,  are  homogeneous  globules  of  living  matter,  without 
organization,  but  endowed  with  the  single  attribute  of  vo- 
luntary motion:  and  even  this  property  was  denied  to  them 
by  some  authors. 

AH  these  fanciful  dreams  have  been  dispelled  by  the  im- 
portant discoveries  of  Ehrenberg,  who  has  recently  found 
that  even  the  Monas  terrno  is  possessed  of  internal  cavities 
for  the  reception  and  the  digestion  of  its  food;  and  who  has 
rendered  it  probable  that  their  organization  is  equally  com- 
plex with  that  of  the  larger  species  of  infusoria,  such  as  the 
Rotifera,  in  which  he  has  succeeded  in  distinguishing  traces 
of  a  muscular,  a  nervous,  and  even  a  vascular  system. 

Those  animalcules,  whose  form  can  be  at  all  distinguished, 
exhibit  a  great  diversity  of  shapes,  and  variety  of  modes  of 
progressive  motion.  Many,  as  the  Cyclidium,  have  the  ap- 
pearance of  a  thin  6val  pellicle,  smoothly  gliding  in  all  di- 
rections through  the  fluid:  some,  as  the  Volvox,  are  globular; 
others,  as  the  Cercaria,  are  shaped  like  a  pear,  tapering  at 
one  end,  and  often  terminating  in  a  slender  tail,  so  as  to  re- 
semble a  tadpole.  In  many,  this  tail  is  of  great  length;  in 
some,  as  the  Furcocerca,  it  is  forked;  in  others,  it  takes  spi- 
ral turns,  like  a  corkscrew.  The  Kerona  has  processes  like 
horns.  The  shape  of  the  Vibrio  is  cylindrical,  and  more 
or  less  pointed  at  one  or  both  ends,  like  an  eel,  or  a  serpent, 
which  animals  it  also  resembles  in  its  undulatory  mode  of 
swimming.*    Some,  as  the  Gonium,  have  an  angular,  others, 

*  Animalcules  refen-ible  to  this  genus  are  met  with  in  great  numbers  in 
blighted  wheat,  (Fig.  2,  p.  58,)  in  sour  paste,  and  in  vinegar  which  has  lost 


INFUSORIA. 


139 


as  the  Kolpoda,  a  waving  outline.    Some,  as  the  Urceolaria 
present  the  likeness  of  a  boll  or  funnel,  and  appear  to  be 
analogous  to  the  Vorticella,  in  which  genus  they  should  pro- 
bably be  included. 

Forms  still  more  irregular  are  exhibited  by  other  infuso- 
ria. Of  these  the  most  singular  is  the  Proteus  (Fig.  78,) 
which  cannot,  indeed,  be  said  to  have  any  determinate  shape, 


78 


79 


for  it  seldom  remains  the  same  for  two  minutes  together.  It 
looks  like  a  mass  of  soft  jelly,  highly  irritable  and  contrac- 
tile in  every  part;  at  one  time  wholly  shrunk  into  a  ball,  at 
another  stretched  out  into  a  lengthened  riband;  and  again, 
at  another  moment,  perhaps,  we  find  it  doubled  upon  itself 
like  a  leech.  If  we  watch  its  motions  for  any  time,  we  see 
some  parts  shooting  out,  as  if  suddenly  inflated,  and  branch- 
ing forth  into  star-like  radiations,  or  assuming  various  gro- 
tesque shapes,  while  other  parts  will,  in  like  manner,  be  as 
quickly  contracted.  Thus  the  whole  figure  may,  in  an  in- 
stant, be  completely  changed,  by  metamorphoses  as  rapid  as 
they  are  irregular  and  capricious. 

The  Volvox  globator,  (Fig.  79)  is  found  in  prodigious 
numbers  at  the  surface  of  many  stagnant  pools.  Its  figure 
is  perfectly  spherical ;  and  its  movements  consist  in  a  conti- 
nual and  rapid  rotation,  round  its  axis,  frequently  remaining 
all  the  while  in  the  same  spot.  Another  species,  the  Volvox 
conjlictor,  moves  by  turning  alternately  to  the  right  and  to 
the  left. 

The  progressive  movements  of  infusory  animalcules  are  of 
two  kinds,  the  one  consisting  in  a  smooth  and  equable  gliding 

the  whole  of  its  alcohol.     In  this  last  fluid  they  sometimes  attain  so  large  a 
size  as  to  be  visible  to  the  naked  eye. 


140  THE  MECHANICAL  FUNCTIONS. 

through  the  fluid,  produced  apparently  by  the  vibrations  of 
cilia,  which  are  set  on  various  parts  of  the  body,  and  often 
seem  to  cover  the  whole  surface:  the  other,  more  rapid  and 
energetic,  when  the  animalcule  darts  forward  in  a  particu- 
lar direction,  as  if  in  pursuit  of  prey,  and  proceeds  by  sudden 
and  irregular  starts,  like  a  vivacious  insect  or  fish.  The  vo- 
luntary nature  of  their  motions  is  evident  from,  the  dexterity 
they  display  in  avoiding  obstacles,  while  swimming  together 
in  myriads  in  a  single  drop. 

The  great  agent  in  the  movements  of  the  animal  frame 
being  the  muscular  fibre,  it  was  natural  to  suppose  that  a 
texture  analogous  to  that  of  muscles  might  exist  in  these 
latter  genera  of  infusoria.  It  was  not  till  very  recently, 
however,  that  the  actual  presence  of  contractile  fibres  could 
be  recognised.  But  this  problem  has  at  length  been  solved 
by  the  discoveries  of  Ehrenberg,  who,  in  his  observations 
of  the  larger  and  more  highly  organized  species  belonging 
to  the  order  of  Rotifera,  has,  with  a  magnifying  power  of 
380,  distinctly  seen  muscular  bands  running  in  pairs  between 
the  two  layers  of  transparent  membrane  which  envelope  the 
body.  When  the  animalcule  throws  itself  into  its  violent 
lateral  contortions,  these  fibrous  bands  are  observed  to  be- 
come broader  and  thicker,  as  well  as  shorter,  on  the  side 
towards  w^hich  the  contractions  take  place.  There  can, 
therefore,  be  no  doubt  that  these  are  muscular  organs,  and 
that  they  are  the  real  agents  by  which  the  motions  wit- 
nessed are  effected. 

These  Rotifera,  or  wheel  animalcules,  ar-e  so  named  from 
{liu^j^  R  their  being  provided  with  an  apparatus 
for  creating  a  perpetual  eddy,  or  circu- 
lar current  in  the  surrounding  fluid. 
The  remarkable  organs,  by  which  this 
effect  is  produced,  are  generally  two  in 
number,  (Fig.  80,  r,  r,)  and  are  situ- 
ated on  the  head,  but  do  not  surround 
the  opening  of  the  mouth,  as  is  the  case 
with  the  tentacula  of  polypes.  They 
consist  of  circular  disks,  the  margins  of 


WHEEL  ANIMALCULES.  Ill 

which  are  fringed  with  rows  of  cilia,  bearing  a  great  resem- 
blance to  a  crown  wheel.     This  wheel  appears  to  be  inces- 
santly revolving,  and  generally  in  one  constant  direction; 
giving  to  the  fluid  a  rotary  impulse,  which  carries  it  round 
in  a  continual  vortex.     The  constancy  of  this  motion  would 
seem  to  indicate  that  it  is  related  to  some  function  of  vital 
importance,  such  as  respiration.     But  even  considered  as 
a  mechanical  action,  which  is  the  view  we  have  now  to  take 
of  it,  this  phenomenon  is  of  a  nature  to  excite  much  curiosi- 
ty; for  the  continued  revolution  round  an  axis  of  any  part 
or  appendage  to  the  body,  is  quite  inconsistent  with  any  no- 
tion we  can  form  of  the  solid  organic  attachment  of  such 
appendage;  and  we  can  have  no  conception  of  organization 
extending  through  the  medium  of  a  fluid,  or  of  any  substance, 
which,  like  a  fluid,  admits  of  the  continual  displacement  of 
its  parts.     M.  Dutrochet  has  offered  an  ingenious  solution 
of  this  difficulty.     He  suggests  that  the  revolution  of  the 
wheels  of  the  Rotifera  may  not  be  real,  but  apparent  only.* 
The  indented  margin  of  each  wheel  being  composed  of  a 
material  so  exceedingly  flexible  as  to  be  capable  of  assuming 
quickly  all  kinds  of  curvatures,  may* be  conceived  to  be 
thrown  into  undulations,  which  follow  one  another  round 
the  circumference;  each  part,  in  succession,  becoming  alter- 
nately convex  and  concave,  and  thus  producing  the  appear- 
ance of  the  actual  advance  of  the  portions  that  are  raised; 
while  their  real  motions  are  only  those  of  elevation  and  de- 
pression, by  the  elongation  and  contraction  of  their  perpen- 
dicular fibres. 

Besides  possessing  extensive  powers  of  locomotion,  the 
infusoria  manifest  in  several  of  the  vital  functions,  as  we 
shall  hereafter  find,  a  degree  of  complication,  which  appears 
to  entitle  them  to  a  higher  station  in  the  animal  scale,  than 
that  which  most  naturalists  have  assigned  to  them.  They 
are  certainly  superior  to  the  sponges  or  polypi,  doomed  by 
nature  to  be  permanently  fixed,  like  plants,  to  the  same  spot; 
and  of  which,  if  we  consider  them  as  compound  beings,  the 

•  The  same  opinion  was  advanced  long  ago  by  Vicq.  d'Azyr. 


142 


THE  MECHANICAL  FUNCTIONS. 


individual  animals  are  often  so  minute  as  to  be  scarcely  vi- 
sible without  the  aid  of  the  microscope.  Mere  size,  indeed, 
is  of  all  the  circumstances  attendant  on  organized  beings,  that 
which  should  least  be  assumed  as  the  criterion  of  complica- 
tion or  refinement  of  structure.  An  object  is  great  or  small, 
only  in  relation  to  the  standard  of  our  own  limited  and 
imperfect  senses;  but  with  reference  to  the  operations  of 
creative  power,  all  such  distinctions  must  vanish.  There 
is  not,  as  far  as  we  have  the  means  of  judging,  in  the  colossal 
fabric  of  the  elephant,  any  structure  more  complicated  than 
exists  in  the  minutest  insect  that  crawls  unheeded  at  our 
feet. 

§  5.  *dcalepha. 


Floating  masses  of  living  gelatinous  matter  are  met  with 
in  every  part  of  the  ocean;  often  in  vast  numbers  and  of  va- 
rious forms;  and  having  but  little  the  appearance  of  belong- 
ing to  the  animal  kingdom.     They  compose  the  order  *^ca- 

lepha,  of  which  the  Medusa 
(Fig.  81)  may  be  taken  as  the 
type.  They  appear,  from  their 
organization,  to  be  raised  but  a 
single  step  above  polypi;  and 
in  point  of  activity  and  loco- 
motive powers,  they  rank 
among  the  lowest  of  those  Zoo- 
phytes which  are  not  perma- 
nently fixed  to  the  spot  where 
they  were  first  developed. 
They  are  almost  wholly  passive 
beings,  floating  on  the  surface 
of  the  sea,  or  remaining  at  a  small  depth  below  it,  carried 
to  and  fro  by  the  motion  of  every  tide  and  current,  and 
destined  to  be  the  unresisting  prey  of  innumerable  tribes 
of  animals  which  people  every  part  of  the  ocean. 

The  usual  form  of  a  Medusa  is  that  of  a  hemisphere,  with 


MEDUSA.  143 

a  marginal  membrane,  like  the  fold  of  a  mantle  extending 
loosely  downwards  from  the  circumference;  together  with  a 
central  pedicle  descending  from  the  lower  surface,  like  the 
stalk  of  a  mushroom,  and  terminating  below  in  several 
fringed  laminae,  or  processes,  which  have  sometimes  been 
denominated  tentacula. 

The  whole  substance  of  the  body  of  these  medusai  is  se- 
mi-transparent and  gelatinous,  without  any  distinct  fibrous 
structure;  yet  it  has  considerable  elasticity,  and  possesses  also 
some  degree  of  contractile  power.  The  animal  is  seen  al- 
ternately to  raise  and  depress  the  margin  of  its  hemispheri- 
cal body,  and  to  flap  with  the  fringed  membrane  or  mantle, 
which  descends  from  it,  in  a  manner  somewhat  similar  to 
the  opening  and  shutting  of  a  parasol.  This  pulsatory  move- 
ment is  performed  about  fifteen  times  in  every  minute,  with 
great  regularity:  and  by  the  reaction  of  the  water,  the  ani- 
mal is  sustained  at  the  surface;  or  by  striking  the  water  ob- 
liquely, it  may  even  perform  a  slow  lateral  movement.  They 
descend  in  the  water  by  simply  contracting  their  dimensions 
in  every  direction.  Sometimes,  in  order  to  sink  more  quick- 
ly, they  turn  themselves  over,  so  that  their  convex  part  is 
undermost. 

Medusae  are  met  with  of  very  various  ^izes;  the  larger 
abound  in  the  seas  around  our  coast;  but  immense  numbers 
of  the  more  minute  and  often  microscopic  species  occur  in 
every  part  of  the  ocean.*  In  some  parts  of  the  Greenland 
seas  they  swarm  to  such  an  extent  that  they  give  a  visible 
tinge  to  the  colour  of  the  waves  for  hundreds  of  miles.  The 
total  number  of  these  animals  dispersed  over  that  space  sur- 
passes the  utmost  stretch  of  the  imagination.  In  these  si- 
tuations a  cubic  foot  of  water,  taken  indiscriminately,  was 
found  by  Mr.  Scoresby  to  contain  above  100,000  of  these 
diminutive  medusae. 

Belonging  to  the  tribe  of  Medusaria  is  a  singular  genus, 

•  The  luminous  property  of  sea  water,  or  its  phosphorescence,  as  it  is  some- 
times called,  generally  arises  from  the  presence  of  minute  medusec,  which 
are  met  with  in  greatest  numbers  at  the  surfiice,  being  spccifially  lighter  than 
the  surrounding  fluid. 


144 


THE  MECHANICAL  FUNCTIONS. 


denominated  the  Beroe,  (Fig.  82  and  83,)  which  is  remark- 
able for  its  organs  of  progressive  motion.  Its  body  is  either 
globular,  or  oblong,  and  it  swims  with  its  aicis  in  a  vertical 
position.     Eight  longitudinal  bands  or  ridges,  which^have 


been  sometimes  compared  to  ribs,  extend  down  its  sides, 
like  those  of  a  melon;  and  along  each  of  these  is  attached  a 
set  of  little  membranes,  extended  horizontally,  and  support- 
ed on  radiating  fibres;  so  that  they  bear  a  pretty  exact  re- 
semblance to  the  fin  of  a  fish.  Their  action  is  not  unlike 
that  of  the  wings  of  a  bird;  for  they  are  made  to  flap  up  and 
down,  striking  the  water  vertically,  and  communicating  an 
ascending  impulse  to  the  body.  This  animal  is  also  pro- 
vided with  two  very  long  and  slender  processes,  which  come 
out  from  the  sides  of  the  body,  and  from  these  a  great  num- 
ber of  still  finer  filaments,  or  cilia,  proceed:  the  whole  ap- 
paratus is  highly  sensitive  and  irritable,  and  on  the  slightest 
touch  the  filaments  are  thrown  into  spiral  coils,  and  retract 
rapidly  within  the  body.  They  thus  act  the  part  of  tenta- 
cula,  or  delicate  organs,  both  of  touch  and  of  prehension.*  It 
was  observed  by  Fabricius,  that  when  a  Beroe  is  cut  into 
many  pieces,  each  piece  continues  to  live,  and  to  swim  about 
by  the  action  of  the  cilia,  which  still  continue  their  vibrato- 
ry motions. 

In  two  other  genera  of  Acalepha,  the  Porpita  and  the 
Velella,  provision  is  made  for  the  mechanical  support  of  the 

*  See  a  description  of  the  Beroe  pikusy  Lam.  by  Dr.  Grant,  in  the  Trans- 
actions of  the  Zoological  Society  of  London,  vol.  i.  p.  9. 


VELELLA.  145 

soft  gelatinous  mass,  by  means  of  an  internal  cartilage.  In 
the  former,  this  cartilage  is  of  a  circular  form;  in  the  latter 
(Fig.  84,)  it  is  oval,  and  bears  upon  its  upper  edge  a  thin 
pellucid  membrane  of  a  triangular  shape,  which  extends  the 
whole  length  of  the  upper  surface  of  the  body.  As  this 
membrane  is  connected  with  the  cartilage  at  its  middle  part 
only,  while  its  edges  are  loose  and  floating,  it  is  peculiarly 
adapted,  when  above  the  surface  of  the  water,  to  catch  the 
wind  and  act  as  a  sail.  Such,  indeed,  appears  to  be  the  pur- 
pose for  which  it  was  given  to  the  animal ;  enabling  it  to  steer 
its  course  by  means  of  the  loose  edges,  and  also  of  the  tenta- 
cula,  which  extend  from  the  lower  side  of  the  body,  and  act 
as  a  rudder,  wdiile  the  sail  is  impelled  by  the  wind. 

A  construction  still  more  artificial  is  provided  in  another 
family  of  the  same  order,  denominated  the  P/iysalida,  or 
Hydrostatic  Acalepha.  They  have  attained  this  latter  ao- 
pellation  from  their  being' rendered  buoyant  by  means  of  ve- 
sicles filled  with  air,  which  enable  them  to  float  without  the 
necessity  of  using  any  exertion  for  that  purpose.  The  Phy- 
salia,  or  Portuguese  Man-of-War,  as  it  is  called,  (Fig.  S5,) 
is  furnished  with  a  large  air-bladder,  of  an  oval  shape,  placed 
on  the  upper  part  of  the  body:  and  also  with  a  membrane 
of  a  beautiful  purple  colour,  which,  as  in  the  Velella,  serves 
as  a  sail.  These  Zoophytes  are  met  with  in  great  numbers 
in  the  Atlantic  Ocean,  and  more  especially  in  its  warmest 
regions,  and  at  a  considerable  distance  from  land.  In  calm 
weather  thl?y  float  on  the  surface  of  the  sea,  rearing  their 
purple  crests,  and.  appearing  at  first  like  large  air  bubbles, 
but  distinguishable  by  the  vivid  hues  of  the  tentacula  which 
hang  down  beneath  them.  Nothing  can  exceed  the  beauty 
of  the  spectacle  presented  by  a  numerous  fleet  of  these  ani- 
mals, quietly  sailing  in  the  tropical  seas.  Whenever  the 
surface  is  ruffled  by  the  slightest  wind,  they  suddenly  absorb 
the  air  from  their  vesicles,  and  becoming  tlius  specifically 
heavier  than  the  water,  immediately  disappear,  by  diving 
into  the  still  depths  of  the  ocean.  By  what  process  they 
effect  these  changes  of  absorption  and  of  reproduction  of  air 
yet  remains  to  be  discovered.     Other  genera,  as  the  Fhys- 

VoL.  I.  19 


146 


THE  MECHANICAL  FUNCTIONS. 


sophora,  have  several  of  these  air-bladders;  but  in  other  re- 
spects resemble  the  ordinary  Medusae,  in  having  no  mem- 
branous crest. 

The  J^ctinix  are  a  tribe  of  Zoophytes,  v^hich,  from  the 
general  resemblance  of  their  forms  to  those  of  Polypi,  are 
b}^  most  naturalists  included  under  that  order.  But  they 
exhibit  a  much  greater  development  in  their  organization; 
having  very  distinct  muscular  fibres,  endowed  with  strong 
powers  of  contraction.  Their  digestive  organs,  also,  as  I 
shall  have  afterwards  occasion  more  fully  to  notice,  are  con- 
structed upon  a  more  complicated  plan  than  in  the  polypus. 
Fig.  86  exhibits  an  Actinia  in  its  contracted  state.  When 
their  tentacula,  which  surround  the  mouth,  and  are  very 
numerous,  are  fully  expanded,  (as  shown  in  Fig.  87,)  these 


animals  present  a  striking  analogy  of  form  to  many  of  the 
compound  flowers;  and  accordingly  the  particular  species 
are  named  from  these  resemblances,  the  sea-anemone^  the 
sea-mar y gold,  the  sea-carnation,  the  sun-flower,  daisy, 
Sic.  Actiniae  are  seen  in  great  numbers  on  many  shores, 
adhering  by  their  flat  surfaces  to  rocks,  and  beinjj  generally 
permanently  fixed  to  their  abode.  When  the  weather  is 
fine,  and  the  sea  calm,  it  is  very  amusing  to  watch  the  rapid 
expansions  and  retractions  of  their  many  coloured  tentacula, 
while  they  are  moving  in  search  of  food:  to  observe  the 
quickness  with  which  they  seize  on  whatever  prey  comes 
within  their  reach,  and  to  notice  the  suddenness  with  which 
they  collapse  into  a  round  contracted  mass,  on  receiving  the 
slightest  injury. 

Yet  these  animals  are  not  of  necessity  confined  to  the  par- 
ticular spots  where  we  see  them  fixed;  for  they  are  capable, 
when  disturbed,  of  seeking,  by  a  slow  progressive  motion, 
a  more  secure  abode.     Reaumur  has  minutely  examined  the 


ECHINODERMATA. 


147 


arrangements  of  their  muscular  fibres,  and  has  described  the 
action  by  which  they  either  attach  themselves  to  the  sur- 
faces of  rocks,  or  effect  their  sluggish  movements.* 


§  G.  Ecldnodermata. 

Ascending  in  the  scale  of  organization  \vc  come  to  the 
Echinoderinata,  a  class  which  comprehends  the  families  of 
the  Jislerida^  the  Echinida,  the  Holothitrida,  and  the  Cri- 
noidea,  together  with  other  tribes  of  less  note. 


These  animals,  both  in  their  general  form,  and  in  the  ar- 
rangement of  their  internal  organs,  retain,  in  a  very  marked 
manner,  the  radiated  disposition  so  characteristic  of  Zoo- 
phytes: for  we  find  all  their  parts  symmetrically  arranged 
either  in  lines,  or  in  compartments,  which  proceed  from  a 
common  centre,  or  axis,  and  which  are  repeated,  in  regular 
succession,  all  round  the  circumference  (See  Fig.  SS  to  94.) 
Besides  an  external  horny,  or  semi-calcareous  covering, 
there  is  also  provided,  for  the  support  of  the  softer  parts,  a 
kind  of  internal  skeleton,  or  jointed  frame-work.  The  or- 
gans in  the  interior  of  the  body  are  farther  supported  by 

*  M^molres  de  TAcacIemie  des  Sciences,  1710,  p.  490. 


148 


THE  MECHANICAL  FUNCTIONS. 


membranous  walls,  which  impart  mechanical  firmness  to  the 
fabric. 

The  Jisterias,  or  star-fish  (Fig.  SS,)  is  so  named  from  its 
star-like  form;  and  the  number  of  rays  composing  the  star 
is  generally  five.  Besides  the  tough  coriaceous  integument, 
which  protects  the  mass  of  the  body,  each  ray  is  farther  sup- 
ported by  a  series  of  calcareous  pieces,  resembling  those 
which  compose  the  spinal  column  of  vertebrated  animals,and 
forming  an  articulated  axis,  constructed  with  the  evident  de- 
sign of  combining  the  greatest  strength  with  a  proper  degree 
of  flexibility.  Cartilaginous  plates  are  also  added  for  the  more 
special  support  of  the  integument.  This  integument  itself 
is  irritable,  and  has  the  power  of  changing  its  form,  although 
the  muscular  fibres  by  w^hich  its  motions  are  effected  are  not 
easily  distinguished.  Calcareous  grains,  of  a  solid  consist- 
ence, are  thickly  interspersed  throughout  its  texture;  and 
these,  in  various  parts  of  the  body,  both  in  the  upper  and 
the  under  side,  often  project  from  the  surface  in  the  form  of 
spines  or  prickles.  They  are  particularly  large  around  the 
mouth  of  the  animal,  which  opens  at  the  centre  of  the  under 
side.  These  calcareous  masses  have  a  crystalline  arrange- 
ment, and  exhibit  in  fracture  the  exact  oblique  angles  cha- 
racteristic of  the  primitive  rhomboid  of  carbonate  of  lime. 

The  under  side  of  each  ray  (Fig.  95)  has  a  groove,  termed, 


95 


9S 


by  Linneus,  the  ambulactnim,  or  avenue,  a  name  which  it 
has  received  from  its  fancied  resemblance  to  a  walk  between 
rows  of  trees;  for  each  groove  contains  a  quadruple  row  of 
perforations,  like  pin  holes,  through  which  small  fleshy 
cylindrical  processes  pass.  These  processes  extend  but  a 
short  distance  from  the  surface;  but  they  admit  of  being 


ECHINUS.  149 

elongated  or  retracted,  at  the  pleasure  of  the  animal,  by  a 
very  curious  mechanism,  which  I  shall  presently  describe. 
By  bending  them  on  either  side,  in  their  expanded  state, 
the  Asterias  is  capable  of  effecting  a  slow  progressive  mo- 
tion; so  that  these  processes  may  be  regarded  as  correspond- 
ing to  feet,  being  levers  for  the  advance  of  the  body.  This, 
it  may  be  remarked,  is  the  first  time  that  we  meet  with  or- 
gans  of  that  description  in  our  progress  through  the  animal 
kingdom.  Each  of  these  feet  is  terminated  by  a  concave 
disk,  which  when  applied  to  any  flat  surface  acts  as  a  sucker, 
on  the  principles  already  adverted  to.''  Reaumur  counted  304 
of  these  feet  in  each  of  the  five  rays  of  the  star  fish,  making 
1520  in  all.t  Each  foot  consists  of  a  tube,  closed  at  the 
outer  end,  and  the  stem  of  which,  after  passing  through  the 
aperture  in  the  integument,  is  dilated  into  a  bag  or  reservoir 
of  fluid;  as  is  shown  in  Fig.  97.  ^  By  the  contraction  of  this 
reservoir,  the  fluid  it  contains  is  propelled  into  the  outer 
portion  of  the  tube,  which  protrudes  by  being  thus  distend- 
ed; the  foot  fixes  itself,  by  means  of  its  terminal  fleshy  disk, 
to  the  point  it  touches,  and  then,  by  retracting,  draws  the 
body  along  for  a  short  distance.  By  the  retreat  of  the  fluid 
into  its  reservoir^  the  foot  is  again  detached,  and  ready 
to  be  moved  forwards,  and  is  thus  made  instrumental  in 
taking  another  step,  by  a  repetition  of  the  same  process.:}: 
From  the  shortness  of  these  feet,  notwithstanding  their  great 

•  Page  105. 

\  !Memoires  de  rAcadcmIc  des  Sciences,  1710,  p.  487. 

\  The  mechanism  by  which  the  feet  arc  protruded  and  retracted  is  illus- 
trated by  the  diagram.  Fig".  97,  which  exhibits  the  bladders  connected  with 
them,  in  different  states  of  distention  and  contraction.  Fig  96  shows  the 
upper  side  of  the  umbulacra,  and  of  the  bladders  comiected  with  the  feet. 
Dr.  Grant,  from  some  observations  which  he  made  on  the  structure  of  the  cilia 
of  the  Beroe  pileus,  is  led  to  suspect  that  the  rapid  vibrations  of  these  sin- 
gular organs  in  the  lowest  animals  may  depend  on  the  undulations  of  water 
conveyed  through  elastic  tubes  along  their  bases,  in  a  manner  resembling 
the  injection  of  the  tubular  tentacula  of  Actinix  and  Astcriae.  If  this  con- 
jecture were  verified,  he  remarks,  one  of  the  most  remarkable  phenomena 
of  animal  motion,  though  one  of  the  most  frequent,  would  lose  much  of  its 
present  marvellous  character. 


150 


THE  MECHANICAL  FUNCTIONS. 


number,  the  advance  which  this  animal  can  make  in  any 
particular  direction  is  excessively  slow. 

Besides  this  movement  of  creeping,  the  Asterias  is  capa- 
ble of  bending  and  unbending  each  of  its  rays:  actions,  how- 
ever, which  it  can  perform  but  very  slowly,  and  not  to  an 
extent  sufficient  to  accomplish  its  removal  from  one  place  to 
another.* 

The  skeleton  of  the  Echinus  or  sea-urchin,  (Fig.  91,)  is 
still  more  artificially  framed  than  that  of  the  Asterias.  It 
has  a  spheroidal  form,  like  that  of  an  orange;  the  calcareous 
material  employed  in  its  construction,  instead  of  forming 
isolated  grains,  is  accumulated  and  extended  into  polygonal 

plates  (Fig.  98,)  the  edges 
of  which  are  dove-tailed 
into  each  other.  The 
form  of  each  piece  is  that 
of  a  lengthened  hexagon; 
and  the  whole  are  regular- 
ly arranged  in  rows,  like 
a  mosaic  or  tesselated  pavement.  Ambulacra  are  also  seen 
on  the  surface  of  the  shell,  passing  vertically  down  the  sides 
of  the  sphere,  similar  to  the  meridians  of  a  globe;  and  con- 
taining, like  those  of  the  Asterias,  a  double  row  of  perfora- 
tions.t 

On  the  outer  spherical  surface  of  the  external  crust,  there 
are  formed  a  great  number  of  calcareous  tubercles,  arranged 
with  beautiful  regularity  and  symmetry  in  double  lines, 
passing,  like  meridian  circles,  fromi  the  upper  to  the  lower 
pole  of  the  sphere.  Each  appears,  when  magnified,  to  be  a 
smooth  and  solid  ball,  projecting  from  the  surface  of  one  of 


*  In  adclltioa  to  these  larger  tubes,  there  exists  also  a  smaller  set,  which 
pierce  the  skin  in  different  places,  and  are  channels  for  the  absorption  of  the 
water  used  in  respiration.  These  I  shall  have  occasion  to  notice  more  parti- 
cularly hereafter. 

■\  An  architecture  of  a  still  more  curious  description  is  exhibited  in  the 
calcareous  frame-work  that  has  been  provided  for  the  support  of  the  teeth, 
and  other  organs  of  mastication,  with  which  this  animal  is  furnished.  The 
etructure  of  these  organs  will  be  noticed  when  treating  of  that  function. 


ECHINUS.  151 

the  polygonal  plates  of  the  crust.  These  balls  serve  for  the 
support  of  the  spines,*  which  have  grooves  or  sockets  at 
their  base,  allowing  of  their  accurate  application  to  the  sphe- 
rical surface  of  the  tubercles.  They  thus  constitute  ball-and 
socket  joints,  allowing  of  free  motion  in  all  directions.  Each 
joint  is  connected  with  the  plate  on  which  it  turns,  by  means 
of  the  integument,  which  acts  the  part  of  a  capsular  ligament; 
and  sets  of  radiating  muscular  fibres  are  provided  for  effect- 
ing the  movements  of  the  spines.  By  employing  these  spines 
as  levers,  the  Echinus  advances  with  great  facility  alono- 
plane  surfaces  at  the  bottom  of  the  sea.  This  animal  is  also 
aided  in  its  progressive  motion  by  the  employment  of 
suckers,  which  are  placed  at  the  end  of  the  slender  tubes, 
protruding  from  the  pores  of  the  ambulacra,  and  analogous 
to  those  of  the  Asterias. 

The  Spatangus,  a  genus  belonging  to  this  order,  buries 
itself  in  the  sand  by  the  action  of  its  spines,  which  on  its 
under  surface  are  short,  thick,  and  expanded  at  the  ends,  like 
the  handle  of  a  spoon,  w^ith  the  convexity  downwards;  and 
which  have  a  limited  rotatory  motion.  Those  which  grow 
from  the  sides  are  more  slender,  and  taper  towards  the  ex- 
tremities, and  when  not  in  use  they  f\dl  flat  upon  the  body 
with  their  points  directed  backwards.  Besides  these,  there 
are  a  few  longer  bristles,  arranged  in  a  crescent  on  the  back, 
and  converging  till  their  points  meet,  but  capable  of  being 
erected  to  a  perpendicular  position.  The  animal,  when 
placed  on  sand,  commences  its  operations  by  revolving  the 
lower  spines,  thus  soon  creating  a  hollow  quicksand,  into 
which  it  sinks  by  its  own  weight  so  far  as  to  enable  the  low- 
est of  the  lateral  spines  to  co-operate  with  them,  by  scatter- 
ing and  throwing  up  the  loosened  particles;  while  these,  at 
the  same  time,  contribute,  by  their  re-action,  still  fiirther  to 
depress  the  body.  As  the  animal  sinks,  a  greater  number 
of  spines  are  brought  into  action,  and  its  progress  becomes 

•  It  has  been  ascertained  by  Mr.  Haiding-er,  that  the  structure  of  these 
spines  is  crystalline,  and  that  their  cleavag^e  presents  the  exact  rhomboidal 
angles  characteristic  of  carbonate  of  lime.  See  his  Translation  of  Mohs's 
Mineralog-y,  vol.  ii.  p.  91. 


152  THE  MECHANICAL  FUNCTIONS. 

more  rapid;  while  the  sand,  that  had  been  pushed  aside, 
flows  back,  and  covers  the  body,  when  it  has  sunk  below 
the  level  of  the  surface.  In  this  situation  the  long  dorsal 
bristles  come  into  play,  preventing  the  sand  from  closing 
completely,  and  preserving  a  small  round  hole  for  the  ad- 
mission of  water  to  the  mouth  and  respiratory  organs.* 

Whenever,  in  following  the  series  of  organic  structures, 
new  forms  are  met  with,  we  always  find  them  accompanied 
by  corresponding  modifications  in  the  processes  of  develop- 
ment. The  organization  of  the  animals  belonging  to  the 
lowest  division  of  the  series  is  not  sufiiciently  perfect  to  af- 
ford the  means,  which  are  supplied  in  the  higher  animals, 
of  removing  or  modifying  the  substances  that  have  at  any 
time  been  deposited,  and  sufiered  to  harden.  Hence  the 
structures  composed  of  these  substances  remain  unchanged 
during  the  life-time  of  the  animal,  although  they  may  conti- 
nue to  receive  additions  of  new  layers  of  the  same  material, 
deposited  upon  their  surface  by  the  soft  parts  in  contact 
with  them;  for  it  is  through  the  medium  of  the  soft  parts 
alone  that  these  materials  are  supplied.  All  the  solid  struc- 
tures of  zoophytes  are  formed  by  this  process,  and  they  are 
subjected  to  all  the  consequences  of  this  law  of  increase.  As 
these  consequences  are  important  in  their  relation  to  the 
conditions  of  growth,  and  to  the  forms  which  result,  it  will 
be  necessary  to  direct  our  attention  to  them  more  particu- 
larly. 

The  influence  which  this  mode  of  increase  by  superficial 
depositions  may  have,  in  changing  the  form  of  the  original 
structure,  will  depend  altogether  upon  the  relative  situations 
of  the  soft  secreting  organ  and  the  hard  part  on  w^iich  it  is 
to  desposite  new  layers:  for,  as  every  new  layer  must  occupy 
the  situation  of  the  soft  organ  which  has  formed  it,  it  must 
displace  the  latter,  and  push  it  back  for  a  space  equal  to  its 
own  thickness.  In  process  of  time,  the  addition  of  numerous 
layers  having  led  to  successive  encroachments  of  the  solid 
substance,  the  latter  will  have  been  displaced  to  an  extent 

The  account  here  given  is  taken  from  Mr.  Osier's  papers  in  the  Philosophi- 
cal Transactions  for  1826,  p.  347. 


ECHINUS.  153 

which  must  sooner  or  later  become  sensible.     If  the  soft 
organs  have  sufBcient  room  for  their  expansion,  as  is  the 
case  when  they  are  external  to  the  hard  axis  of  the  zoophyte, 
the  growth  of  that  axis  may  go  on  without  impediment;  and 
no  change  need  take  place  in  the  general  figure  of  the  parts, 
since  their  relative  proportions  and  situations  may  be  pre- 
served unaltered.     But  this  cannot  happen  when  the  new 
materials  are  to  be  deposited  on  the  internal  surface  of  a 
membrane,  or  a  shell,  which  completely  encloses  the  soft 
parts:  for  the  additions  thus  made  to  the  thickness  of  the 
layer  must  encroach  upon  the  space  within;  and,  that  space 
being  limited,  the  soft  parts  contained  in  it  will  not  merely 
cease  to  grow,  but  will  be  actually  contracted  in  their  dimen- 
sions: and  if  the  process  of  deposition  were  to  go  on,  the 
space  occupied  by  the  soft  organs  would  at  last  be  entirely 
filled  up  with  solid  matter,  and  the  cavity  be  obliterated. 
Accordingly  it  is  necessary,  whenever  cells,  intended  for 
the  lodgement  of  soft  organs,  are  to  be  constructed  of  hard 
materials,  that  the  foundation  of  these  cells  should  be  laid, 
and  their  construction  begun,  upon  a  scale  of  the  same  size 
as  that  which  they  are  intended  to  have  at  all  future  periods; 
because,  as  we  have  just  seen,  after  the  innermost  layer  has 
been  deposited,  they  admit  not  of  any  future  enlargement 
of  their  cavity.     Thus,  we  find  that,  in  the  case  of  polypes 
which  are  lodged  in  cells,  the  walls  of  these  cells  must  be 
completed  before  the  soft  polypoMS  portion  has  attained  its 
full  expansion;  for  were  it  at  first  built  of  a  smaller  size, 
proportioned  to  that  of  the  young  polype,  it  would  prevent 
all  farther  growth. 

The  globular  shell  of  the  Echinus,  which  is  external  to 
the  soft  parts  that  nourish  it,  and  which  yet  grows  from  a 
very  minute  sphere  to  one  of  large  dim.ensions,  kee])ing  pace 
with  the  gradual  expansion  of  the  internal  organs,  might  ap- 
pear to  be  an  exception  to  the  general  law.  Nature  has, 
however,  accomplished  her  purpose  without  deviating  from 
her  usual  plan;  first,  by  dividing  the  shell  of  the  Echinus  into 
a  great  number  of  small  pieces;  and  secondly,  by  giving  to 
each  piece  the  polygonal  form,  which  is  best  adapted  to  their 

Vol.  I.  20 


154  THE  MECHANICAL  FUNCTIONS. 

mutual  and  perfect  junction,  without  leaving  any  intervening 
spaces.  Thus,  has  she  provided  for  the  enlargement  of  the 
whole  structure,  by  admitting  of  additions  being  made  to 
the  margins  of  each  of  the  separate  polygonal  pieces;  fresh 
layers  of  calcareous  substance  be^jng  deposited  on  the  under 
side,  and  on  the  edges  of  each,  in  proportion  as  the  expan- 
sion of  the  contents  of  the  shell  causes  their  separation. 
That  such  a  succession  of  deposites  has  taken  place,  may  easi- 
ly be  seen,  by  minutely  examining  the  texture  of  the  plates, 
which  will  be  found  marked  by  concentric  polygonal  lines. 
(Fig.  99.) 

The  spines  of  the  Echinus  must  be  formed  by  the  succes- 
sive deposition  of  layers  on  their  outer  surface,  as  appears 
from  the  examination  of  their  structure,  when  a  longitudinal 
section  of  them  has  been  made.  The  lines  exhibiting  the 
succession  of  layers  are  seen  in  Fig.  100,  which  represents 
such  a  section.  Hence,  they  are  probably  deposited  by  the 
membrane  which  covers  them  during  the  whole  period  of 
their  growth. 

There  is  probably  no  series  of  animals  that  exemplify  in 
so  marked  a  manner  as  the  Echinodermata,  the  gradations 
which  nature  has  observed  in  passing  from  one  model  of 
construction  to  another  of  a  totally  different  aspect,  through 
every  intermediate  form.  What  shapes  can  be  more  diver- 
sified, and  apparently  irreducible  to  a  common  standard, 
than  those  of  the  star-like  Asterias,  (Fig.  SS)  of  the  globu- 
lar Echinus,  (Fig.  91,)  and  of  the  lily-shaped  Pentacrinus; 
(Fig.  94,)  and  yet  we  find  these  passing  the  one  into  the 
other  by  the  most  gradual  transitions?  Setting  out  from 
the  star  with  five  slender  rays,  which  is  the  standard  form 
of  the  Asterias;  we  find  the  rays,  in  succeeding  species,  as- 
suming gradually  a  greater  breadth  at  their  base,  and  their 
sides  joining  at  more  obtuse  angles:  the  star-like  form  is 
gradually  effaced,  and  the  outline  is  rather  a  pentagon,  with 
its  sides  curved  inwards  (Fig.  89.)  We  soon  perceive  this 
curvature  giving  place  to  a  straight  line,  so  that  the  shape 
becomes  an  exact  pentagon.  The  next  change  effected  is  in 
the  angles  of  this  pentagon,  which  by  degrees  are  lost  in  a 


ECHINUS.  155 

general  rounded  outline;  still,  however,  preserving  its  flat- 
ness.    This  stage  is  attained  in  the  Scutella,  and  the  C/y- 
jnaster.     (Fig.  90.)     We  next  find  that,  in  the  Spatangus, 
the  thickness  increases;  though  at  first  with  an  oval  outline, 
and  with  several  changes  in  the  situation  of  the  mouth  of  the 
animal.     At  length,  after  passing  through  many  intermediate 
steps,  we  arrive  at  the  perfectly   circular  and  spheroidal 
Echinus.     (Fig.  91.)     If  we  might  he  permitted  to  conjec- 
ture the  objects  of  all  these  changes,  which  occur  in  this  con- 
tinuous gradation,  we  might  not  unreasonably  suppose  them 
to  be  the  concentration  of  the  internal  organs  into  one  com- 
pact mass,  and  the  retrenchment  of  all  the  external  appen- 
dages.    It  is  also  curious  to  observe,  how,  amidst  all  these 
modifications,  the  double  rows  of  perforations,  which  con- 
stitute the  ambulacra,  retain  their  situations,  diverging  in 
five  equidistant  lines  from  one  of  the  extremities  of  the  axis, 
and  winding  round  to  the  other. 

Returning  to  the  Asterias,  we  can  trace  changes  equally 
gradual,  though  in  an  opposite  sense,  in  another  series,  which 
presents  a  striking  contrast  with  the  former.     Here,  instead 
of  the  retrenchment  of  the  appendages,  we  find  them  great- 
ly developed,  and  amplified  in  every  possible  degree.     The 
rays  of  the  Asterias  become  narrower,  while  their  length  is 
at  the  same  time  increased;  the  vital  organs,  and  also  the 
tubular  feet,  are  gradually  withdrawn  from  them,  and  retire 
within  a  central  disk,  to  which  the  slender  rays,  now  bereft 
of  feet,  become  mere  appendages.     Such  is  the  condition  of 
the  Ophiura.     (Fig.  92.)     By  the  prolongation  and  taper- 
ing of  these  rays  to  slender  filaments,  they  acquire  a  greater 
prehensile  power,  and  twine  with  ease  round  their  prey. 
We  next  find  their  number  augmented;  it  is  at  first  dou- 
bled, then   tripled,  and  at  lengtli    indefinitely   augmented. 
They  also  become  branched,  subdividing  by  simple  bifurca- 
tions, as  in  the  Euryale palmiferiim  (Fig.  93;)  next  into 
minuter  ramifications,  as  in  the  Caput  Medusic,  where  the 
thousands  of  filaments  have  the  appearance  of  a  tangled  web, 
which  defies  all  attempts  at  unravelling. 

The  steps  are  but  short  from  the  Comatula  to  the  Crinoi- 


156  THE  MECHANICAL  FUNCTIONS. 

dea,  or  lily-shaped  tribe,  (of  which,  Fig.  94,  representing 
the  Pentacrirms  europxuSj  is  an  example;)  for  they  consist 
chiefly  in  the  addition  of  a  jointed  stalk,  which  is  made  to 
proceed  downwards  from  the  centre  of  the  whole  assem- 
blage of  rays,  and  which  is  to  serve  as  a  common  stem  for 
sustaining  the  whole  mass;  while  the  branches  themselves 
are  carried  up,  and  folded  inwards.  The  lower  joint  of  the 
foot-stalk  is  a  little  expanded,  in  order  to  procure  a  more 
extensive  base  of  support;  and  the  whole  structure  thus  pre- 
sents a  remarkable  resemblance  to  a  liliaceous  plant. 


(     157     ) 


CHAPTER  III. 


MOLLUSCA. 

§  1.  Molliisca  in  general. 

The  series  of  animal  structures,  arranged  according  to 
their  mechanical  functions,  conducts  us  next  to  the  JMoUus- 
ca;  an  assemblage  of  beings  which  was  first  recognised  as 
constituting  one  of  the  primary  divisions  of  the  animal  king- 
dom by  Cuvier,  the  greatest  naturalist  of  modern  times.  A 
vast  multitude  of  species,  possessing  in  common  many  re- 
markable physiological  characters,  are  comprehended  in  this 
extensive  class.  In  all,  as  their  name  imports,  the  body  is 
of  soft  consistence;  and  it  is  enclosed  more  or  less  com- 
pletely in  a  muscular  envelope,  called  the  mantle,  composed 
of  a  layer  of  contractile  fibres,  which  are  interwoven  with 
the  soft  and  elastic  integument.  Openings  are  left  in  this 
mantle  for  the  admission  of  the  external  fluid  to  the  mouth 
and  to  the  respiratory  organs,  and  also  for  the  occasional  pro- 
trusion of  the  head  and  the  foot,  when  these  organs  exist. 
But  a  large  proportion  of  the  animals  of  this  class  arc  ace- 
phalous, that  is,  destitute  of  a  head,  and  the  mantle  is  then 
occasionally  elongated  to  form  tubes,  often  of  considerable 
length,  for  the  purpose  of  conducting  water  into  the  interior 
of  the  body. 

Mollusca,  with  the  exception  of  a  few  among  the  higher 
orders,  are  but  imperfectly  furnished  with  organs  of  loco- 
motion. The  greater  number,  indeed,  are  formed  for  an 
existence  as  completely  stationary  as  the  Zoophytes  attaclied 
to  a  fixed  base.  The  Oyster,  the  Muscle,  and  the  Limpet,  for 
example,  arc  usually  adherent  to  rocks  at  the  bottom  of  the 
sea,  and  are  consequently  dependent  for  their  nourishment 


185  THE  MECHANICAL  FUNCTIONS. 

on  the  supplies  of  food  casually  brought  within  their  reach 
by  the  waves  and  currents  of  the  ocean.  This  permanent 
attachment  to  the  solid  body  on  which  they  fix  their  abode, 
does  not,  however,  take  place  till  they  have  arrived  at  a 
certain  period  of  their  growth:  for  at  the  commencement  of 
their  separate  existence,  that  is,  immediately  after  they  are 
hatched,  they  are  free  to  move  in  the  water,  and  to  roam 
in  search  of  a  habitation.  In  this  respect,  therefore,  they 
preserve  an  analogy  with  the  gemmules  of  sponges,  and  of 
polypi,  which  exercise  locomotive  powers  only  in  the  ear- 
ly stages  of  their  devlopment.* 

The  organization  of  the  Mollusca  being  unfitted  for  the 
construction  of  an  internal  skeleton.  Nature  has  ordained 
that  the  purposes  of  mechanical  support  and  protection  shall 
be  answered  by  the  formation  of  hard  calcareous  coverings, 
or  shells,  the  result  of  a  peculiar  process  of  animal  production. 
These  shells  are  formed  either  of  one  piece,  or  of  several; 

*  This  analogy  is  strengthened  by  the  circumstance  that  the  movements 
of  many  of  these  animals,  in  the  first  periods  of  their  existence,  are  effected 
by  the  same  mechanismof  vibratory  cilia  which  we  found  to  be  instrumental 
in  the  progression  of  the  infusory  animalcules,  and  of  the  young  of  polypi. 
On  observing  the  first  evolution  of  the  ova  of  the  Buccinum  undaium.  Dr. 
Grant  found  them  to  consist  of  groups  of  spherical  gelatinous  bodies,  which 
soon  become  covered  on  one  side  with  a  transparent  envelope,  the  rudiment 
of  the  future  shell;  while,  on  the  other  side,  the  gelatinous  matter  is  extend- 
ed outwards,  so  as  to  form  the  margin  of  an  internal  cavity,  of  which  the  en- 
trance is  surrounded  with  vibratory  cilia,  and  in  the  interior  of  which  a  revo- 
lution of  particles  is  seen,  indicating  a  constant  current  of  fluid.  The  vibra- 
tions of  these  ciha  are  perceived  long  before  the  pulsations  of  the  heart,  and 
even  before  any  appearance  of  that  organ  is  visible;  th(jy  are,  indeed,  the 
first  indications  of  life  in  the  embryo.  The  cilia  are  in  activity  even  before 
the  animal  is  hatched:  for  while  confined  within  the  q^^,  it  is  seen  almost 
continually  revolving  around  its  centre;  amotion  which  appears  destined  to 
bring  a  constant  supply  and  renewal  of  sea  water  into  the  interior  of  the  or- 
ganization, in  order  to  perfect  the  formation  of  the  shell  before  the  animal 
is,  as  it  were,  launched  into  the  ocean.  Possibly,  also,  the  continued  friction 
of  the  cilia  against  the  interior  of  the  e^g  may  tend  to  abrade  it,  and  open  a 
passage  for  the  young  animal.  No  sooner  has  the  animal  effected  its  escape, 
than  it  darts  rapidly  forwards  by  the  motion  of  its  cilia.  The  same  appear- 
ances have  also  been  observed  by  Dr.  Grant  in  the  young  of  different  Mol- 
lusca, such  as  the  Dwis,  Eolis,  &c.,  which  have  no  shell.— Edin.  Journal  of 
Science,  Vol.  vii. 


MOLLUSCA. 


159 


the  separate  pieces,  in  cither  case,  being  termed  valves;  so 
that  shells  may  be  either  univalve,  bivalve,  or  miiltivalve, 
according  as  they  consist  of  one,  two,  or  more  pieces.  Uni- 
valve shells  have  generally  more  or  less  of  a  spiral  form, 
and  are  then  called  turbinated  shells.  In  a  few,  the  cavity 
of  the  shell  is  divided  by  transverse  partitions  into  nume- 
rous compartments.  Some  Mollusca  have  internal  shells  for 
the  defence  and  support  of  particular  organs;  and  others 
have  shells  which  are  partly  external,  and  partly  internal. 
As  respects  their  shape,  colour,  and  appearance,  shells  ad- 
mit of  infinite  diversity;  yet,  as  will  presently  be  shown, 
all  are  composed  of  the  same  kind  of  material;  and  their 
production  and  increase  are  regulated  by  the  same  uniform 
laws. 


§  2.  Jlcejihala. 

The  Mollusca  which  inhabit  bivalve  shells,  such  as  the 
Oyster,  the  Muscle,  and  the  Cockle,  are  all  acephalous. 
The  two  valves  of  the  shell  are  united  at  the  back  by  a 
hinge  joint,  often  very  artificially  constructed,  having  teeth 
that  lock  into  each  other:  and  the  mechanism  of  this  arti- 
culation varies  much  in  different  species.  The  hino-e  is  se- 
cured by  a  substance  of  great  strength.     It  is  seen  in  VW. 

101,  which  shows  the  valves 
of  the  Unio  batava,  with  the 
connecting  ligament.  This 
ligament  is  composed  of  two 
kinds  of  texture:  the  one, 
which  is  always  external,  is 
strictly  ligamentous;  that  is, 
perfectly  inelastic:  the  other 
has  more  of  the  properties  of 
cartilage,  being  highly  elastic, 
and  formed  of  parallel  series 
of  condensed  transverse  fibres, 
101  directed  from  the  hinire  of  one 
valve  to  the  similar  part  of  the  other,  and  having  generally 


160 


THE  MECHANICAL  FUNCTIONS. 


a  deep  black  colour,  and  a  pearly  lustre.  The  cartilage  is 
always  situated  within  the  ligament,  sometimes  in  immediate 
contact,  and  forming  with  it  one  of  the  same  mass:  at  other 
times  placed  at  a  distance,  in  a  triangular  cavity,  amongst 
the  teeth  of  the  hinge.  The  closing  of  the  valves  produces, 
in  all  cases,  a  compression  of  the  cartilage,  the  elasticity  of 
which  tends,  therefore,  to  separate  the  valves  from  each 
other;  that  is,  to  open  the  shell. 

Durins:  the  life  of  the  animal,  the  usual  and  natural  state 
of  its  shell  is  that  of  being  kept  open  for  a  little  distance, 
so  as  to  allow  of  the  insfress  and  esiress  of  the  water  neces- 
sary  for  its  nourishment  and  respiration.  But  as  a  security 
against  danger,  it  was  necessary  to  furnish  the  animal  with 
the  means  of  rapidly  closing  the  shell,  and  retaining  the 
valves  in  a  closed  state.  These  actions  being  only  occa- 
sional, yet  requiring  considerable  force,  are  effected  by 
a  muscular  power:  for  which  purpose  sometimes  one,  some- 
times two,  or  even  a  greater  number,  of  strong  muscles  are 
placed  between  the  valves,  their  fibres  passing  directly 
across  from  the  inner  surface  of  the  one  to  that  of  the  other, 

and  firmly  attached  to  both. 
— They  are  named,  from 
their  office  of  bringing  the 
valves  towards  each  other, 
the  adductor  muscles.  Fig. 
102,  which  represents  the  sec- 
tion of  an  oyster,  shows  the 
situation  of  the  hinge  l,  the 
adductor  muscles  a,  and  the  transverse  direction  of  its  fibres, 
with  respect  to  the  valves.  When  these  muscles  are  not  in 
action,  the  elasticity  of  the  cartilage  attached  to  the  hinge  is 
sufficient  to  separate  the  valves;  but  as  they  were  not  intend- 
ed to  open  beyond  a  certain  extent,  it  was  necessary  to  pro- 
vide some  limitation  to  the  action  of  the  cartilage.  The  ad- 
ductor muscle  might,  it  is  evident,  be  called  into  play  to 
counteract  that  action;  but  this  would  require  a  constant 
muscular  exertion,  and  a  great  expenditure,  therefore,  of 
vital  force.    Nature  has  always  shown  a  solicitude  to  econo- 


MOLLUSCA  ACEPHALA. 


161 


103 


mize  muscular  power,  whenever  a  substitute  could  be  had, 
and  such  a  substitute  she  has  here  provided,  by  uniting  with 
the  muscle  an  elastic  ligament,  of  a  peculiar  construction. 
It  has  a  texture  similar  to  that  of  the  ligamentum  nuchw^ 
and  being  placed  on  the  side  of  the  muscle  next  to  the  hinge, 
allows  the  valves  to  separate  to  the  proper  distance  only.* 
When  the  animal  dies,  the  muscular  force  ceases,  but  the  li- 
gament with  which  the  muscle  is  associated,  retaining  its  elas- 
ticity, allows  the  shell  to  open,  but  only  to  a  certain  extent; 
and,  accordingly,  this  is  the  state  in  which  we  find  bivalve 
shells  that  are  cast  upon  the  shore,  after  the  soft  flesh  of  the 
animal  has  decayed  and  been  washed  out,  provided  the  car- 
tilage and  the  ligament  of  the  hinge  are  still  preserved.! 

The  simple  actions  of  opening  and  closing  the  valves  arc 
capable  of  being  converted  into  a  means  of  retreating  from 

danger,  or  of  removing  to  a  more  com- 
modious situation,  in  the  case  of  those 
bivalves  which  are  not  actually  attached 
to  rocks  or  other  fixed  bodies.  Dique- 
mare  long;  asro  observed  that  even  the 
oyster  has  some  power  of  locomotion,  by 
suddenly  closing  its  shell,  and  thereby 
expelling  the  contained  water,  with  a  de- 
gree of  force,  which  by  the  reaction  of 
the  fluid  in  the  opposite  direction,  gives 
a  sensible  impulse  to  the  heavy  mass.  He 

t 

*  This  remarkable  structure  was  first  described  by  Dr.  'Leach,  in  a  paper 
read  before  the  Royal  Academy  of  Paris.  Bulletin  des  Sciences,  1818,  p. 
14.     See  also  Gray,  in  Zoolog-ical  Journal,  I.  219. 

\  The  Pholas  is  an  exception  to  this  rule;  for  instead  of  its  valves  being- 
united,  as  usual,  by  an  elastic  ligament,  they  are  connected  chiefly  by  means 
of  muscles.  This  departure  from  the  ordinary  structure  is  probably  occa- 
sioned by  a  new  condition  introduced  into  the  economy  of  the  animal  in  con- 
-  sequence  of  its  being-  fitted  for  excavating  passages  through  liai-d  rocks.  It 
is  furnished,  for  this  purpose,  with  a  complicated  boring  apparatus  moved 
by  many  muscles,  and  requiring  great  freedom  of  action.  Fig.  103  repre- 
sents the  shell  of  the  Fholas  Candida  extremely  expanded,  in  order  to  show 
the  hinge,  together  with  the  ligament,  l;  the  long  and  thin  process  of  shell, 
p,  to  the  ends  of  which,  on  each  side,  a  pair  of  fan-shaped  muscles,  more  par- 
ticularly employed  in  boring,  are  attached^  and  the  two  adductor  muscles. 
Vol.  I.  21 


162  THE  MECHANICAL  FUNCTIONS. 

notices  the  singular  fact  that  oysters,  which  are  attached  to 
rocks  occasionally  left  dry  by  the  retreat  of  the  tide,  always 
retain  within  their  shells  a  quantity  of  water  sufficient  for 
respiration,  and  that  they  keep  the  valves  closed  till  the  re- 
turn of  the  tide:  whereas  those  oysters  which  are  taken  from 
greater  depths,  where  the  w^ater  never  leaves  them,  and  are 
afterwards  removed  to  situations  where  they  are  exposed  to 
these  vicissitudes,  of  which  they  have  had  no  previous  ex- 
perience, improvidently  open  their  shells  after  the  sea  has 
left  them,  and  by  allowing  the  water  to  escape,  soon  perish.* 
Many  bivalve  mollusca  are  provided  with  an  instrument 
shaped  like  a  leg  and  foot,  which  they  employ  extensively 

for  progressive  motion.  Its  form 
in  the  Cardium,  or  cockle,  is  seen 
in  Fig.  ]  04.  This  organ  is  com- 
posed of  a  mass  of  muscular  fibres, 
interwoven  together  in  a  very  com- 
plex manner,  and  which  may  be 
compared  to  the  muscular  structure 
of  the  human  tongue:  the  effect  in  both  is  the  same,  namely, 
the  conferring  a  power  of  motion  in  all  possible  ways;  thus 
it  may  be  readily  protruded,  retracted,  or  inflected  at  every 
point.  The  Solen,  or  razor-shell  fish,  has  a  foot  of  a  cylin- 
drical shape,  tapering  at  the  end,  and  much  more  resembling 
in  its  form  a  tongue  than  a  foot.  In  some  bivalves  the  dila- 
tation of  the  foot  is  effected  by  a  curious  hydraulic  mechan- 
ism: the  interior  of  the  organ  is  formed  of  a  spongy  texture, 
capable  of  receiving  a  considerable  quantity  of  water,  which 
the  animal  has  the  powder  of  injecting  into  it,  and  of  thus  in- 
creasins;  its  dimensions. 

The  foot  of  the  Mytihis  edid'is,  or  common  muscle,  can 
be  advanced  to  the  distance  of  two  inches  from  the  shell, 
and  applied  to  any  fixed  body  within  that  range.     By  at- 

A  A,  which  retain  the  valves  in  contact  independently  of  the  ligaments.     For 
a  full  description  of  this  apparatus,  I  must  refer  to  a  paper  by  Mr.  Osier,  on 
burrowing  and  boring  marine  animals,  contained  \n  the  Phil.  Trans,  for  1826, 
p.  342,  from  which  tlie  above  figure  has  been  taken. 
*  Journal  do  Physique,  xxviii.  244. 


MOLLUSCA  ACEPHALA.  163 

taching  the  point  to  such  hody,  and  retracting  the  foot,  this 
animal  drags  its  shell  towards  it;  and  by  repeating  the  ope- 
ration successively  on  other  points  of  the  fixed  object,  con- 
tinues slowly  to  advance. 

This  instrument  is  of  great  use  to  such  shell-fish  as  conceal 
themselves  in  the  mud  or  sand,  which  its  structure  is  then 
peculiarly  adapted  for  scooping  out.  The  Cardium  conti- 
nually employs  its  foot  for  this  purpose:  first  elongating  it 
and  directing  its  point  downwards,  and  insinuating  it  deep 
into  the  sand;  and  next,  turning  up  the  end,  and  forming  it 
into  a  hook,  by  which,  from  the  resistance  of  the  sand,  it  is 
fixed  in  its  position,  and  then  the  muscles  which  usually  re- 
tract it  are  thrown  into  action,  and  the  whole  shell  is  alter- 
nately raised  and  depressed,  moving  on  the  foot  as  on  a  ful- 
crum. The  effect  of  these  exertions  is  to  drag  the  shell 
downwards.  When  the  animal  is  moderately  active  these 
movements  are  repeated  two  or  three  times  in  a  minute. 
The  apparent  progress  is  at  first  but  small;  the  shell,  which 
was  raised  on  its  edge  at  the  middle  of  the  stroke,  falling 
back  on  its  side  at  the  end  of  it;  but  when  the  shell  is  bu- 
ried so  far  as  to  be  ^supported  on  its  edge,  it  advances  more 
rapidly,  sinking  visibly  at  every  stroke,  till  nothing  but  the 
extremity  of  the  tube  can  be  perceived  above  the  sand.  JNIr. 
Osier,  who  has  given  us  this  account,*  observes  that  the  in- 
stinct, which  directs  the  animal  thus  to  procure  a  shelter, 
operates  at  the  earliest  period  of  its  existence.  Tlie  Mija 
iruncata,  when  fully  grown,  will  not  attempt  to  burrow; 
but  on  placing  two  young  ones,  which  were  scarcely  more 
than  a  line  in  length,  and  apparently  but  just  excluded,  on 
sand,  in  a  glass  of  sea-water,  he  found  that  they  buried  them- 
selves immediately. 

By  a  process  exactly  the  inverse  of  this,  that  is,  by  dou- 
bling up  the  foot,  and  pushing  with  it  downwards  against  the 
sand  below,  the  shell  may  be  again  made  to  rise  by  the  same 
kind  of  efforts  which  before  protruded  the  foot.  By  this 
process  of  burrowing  the  animal  is  enabled  quickly  to  retreat 

*  Phllos.  Trans,  for  1826,  p.  349. 


164  THE  MECHANICAL  FUNCTIONS. 

when  clanger  presses:  and  when  this  is  past,  it  can,  with 
equal  facility,  emerge  from  its  hiding  place. 

The  Cardium  can  also  advance  at  the  bottom  of  the  sea 
along  the  surface  of  the  soft  earth,  pressing  backwards  with 
its  foot,  as  a  boatman  impels  his  boat  onwards,  by  pushing 
with  his  pole  against  the  ground,  in  a  contrary  direction.  It 
is  likewise  by  a  similar  expedient  that  the  Solen  forces  its 
way  through  the  sand,  expanding  the  end  of  its  foot  into  the 
form  of  a  club.  The  course  of  these  locomotive  bivalves 
may  readily  be  traced  on  the  sand  by  the  furrows  which 
they  plough  up  in  their  progress. 

This,  as  well  as  many  other  of  the  bivalve  mollusca,  are 
enabled  by  the  great  size  and  flexibility  of  this  organ  to 
execute  various  other  movements,  of  which,  from  the  habit- 
ual inactivity  of  animals  of  this  class,  we  should  scarcely  have 
supposed  them  capable.  The  Tellina  is  remarkable  for  the 
quickness  and  agility  with  which  it  can  spring  to  considera- 
ble distances  by  first  folding  the  foot  into  a  small  compass, 
and  then  suddenly  extending  it;  while  the  shell  is  at  the 
same  time  closed  with  a  loud  snap. 

The  Pinna,  or  Marine  Muscle,  when  inhabiting  the  shores 
of  tempestuous  seas,  is  furnished,  in  addition,  with  a  singu- 
lar apparatus  for  withstanding  the  fury  of  the  surge,  and  se- 
curing itself  from  dangerous  collisions,  which  might  easily 
destroy  the  brittle  texture  of  its  shell.  The  object  of  this 
apparatus  is  to  prepare  a  great  number  of  threads,  which  are 
fastened  at  Various  points  to  the  adjacent  rocks,  and  then 
tightly  drawn  by  the  animal;  just  as  a  ship  is  moored  in  a 
convenient  station  to  avoid  the  buffeting  of  the  storm.  The 
foot  of  this  bivalve  is  cylindrical,  and  has,  connected  with 
its  base,  a  round  tendon  of  nearly  the  same  length  as  itself, 
the  office  of  which  is  to  retain  all  the  threads  in  firm  adhe- 
sion with  it,  and  concentrate  their  powxr  on  one  point.  The 
threads  themselves  are  composed  of  a  glutinous  matter,  pre- 
pared by  a  particular  organ.  They  are  not  spun  by  being 
drawn  out  of  the  body  like  the  threads  of  the  silk-worm,  or 
of  the  spider,  but  they  are  cast  in  a  mould,  when  they  hard- 
en, and  acquire  a  certain  consistence  before  they  are  em- 


MOLLUSCA  ACEPHALA.  165 

ployed.     This  mould  is  curiously  constructed;  there  is  a 
deep  groove  which  passes  along  the  foot  from  tlic  root  of  the 
tendon  to  its  other  extremity;  and  the  sides  of  this  groove 
are  formed  so  as  to  fold  and  close  over  it,  thereby  convert- 
ing it  into  a  canal.    The  glutinous  secretion,  which  is  poured 
into  this  canal,  dries  into  a  solid  thread;  and  when  it  has  ac- 
quired sufficient  tenacity,  the  foot  is  protruded,  and  the 
thread  it  contains  is  applied  to  the  object  to  which  it  is  to  be 
fixed;  its  extremity  being  carefully  attached  to  the  solid  sur- 
face of  that  object.     The  canal  of  ^thc  foot  is  then  opened 
along  its  whole  length,  and  the  thread,  which  adheres  by  its 
other  extremity  to  the  large  .tendon  at  the  base  of  the  foot, 
is  disengaged  from  the  canal.     Lastly,  the  foot  is  retracted, 
and  the  same  operation  is  repeated. 

Thread  after  thread  is  thus  formed,  and  applied  in  diffe- 
rent directions  around  the  shell.     Sometimes  the  attempt 
fails  in  consequence  of  some  imperfection  in  the  thread;  but 
the  animal,  as  if  aware  of  the  importance  of  ascertainino-  the 
strength  of  each  thread,  on  which  its  safety  depends,  tries 
every  one  of  them  as  soon  as  it  has  been  fixed,  by  swingino" 
itself  round,  so  as  to  put  it  fully  on  the  stretch:  an  action 
which  probably  also  assists  in  elongating  the  thread.     When 
once  the  threads  have  been  fixed,  the  animal  does  not  ap- 
pear to  have  the  power  of  cutting  or  breaking  them  off.     The 
liquid  matter  out  of  which  they  are  formed  is  so  exceedino-- 
ly  glutinous  as  to  attach  itself  firmly  to  the  smoothest  bo- 
dies.    It  is  but  slowly  produced,  for  it  appears  that  no  Pin- 
na is  capable   of  forming  more  than  four,  or  at  most  five 
threads  in  the  course  of  a  day  and  night.     The  threads  that 
are  formed  in  haste,  when  the  animal  is  disturbed  in  its  ope- 
rations, are  more  slender  than  those  tliat  are  constructed  at 
its  leisure,     Reaumur,  to  whom  we  are  indebted  for  these 
interesting  observations,  states,  also,  that  the  marine  muscles 
possess  the  art  of  forming  these  threads  from  the  earliest  pe- 
riods of  their  existence:  for  he  saw  them  practising  it,  when 
the  shells  in  whicli  they  were  enclosed  were  not  larger  than 
a  millet  seed.^     In  Sicily,  and  other  parts  of  the  Mediter- 

•  Memoircs  de  I'Acadcmic  des  Sciences:  1711,  p.  118  to  123.    Poli  con- 


166 


THE  MECHANICAL  FUNCTIONS. 


ranean,  these  threads  have  been  manufactured  into  gloves, 
and  other  articles,  which  resemble  silk. 


§  3.  Gasteropoda, 

The  Mollusca  which  inhabit  univalve  or  turbinated  shells, 
belong  to  the  order  of  Gasteropoda,  and  have  a  more  high- 
ly developed  organization  than  the  Acephala.  The  part 
which  performs  the  office  of  a  foot  is  a  broad  expansion  of 
fleshy  substance,  occupying  nearly  the  whole  under  surface 
of  the  animal,  and  forming  a  flat  disk,  capable  of  being 

applied  to  the  plane  along 
which  it  moves.  This  is 
seen  in  the  Planorbis  (Fig. 
105,  D.)  In  some  species  it 
is  fashioned  into  a  project- 
ing ridge  which  cuts  its  way, 
like  a  ploughshare,  along  the  surface  on  which  it  moves. 
The  bands  of  muscular  fibres,  which  compose  the  principal 
part  of  its  structure,  are  short,  and  are  interlaced  together 
in  a  very  intricate  arrangement.  All  the  columns  of  their 
fibres  terminate  atM;he  surface  of  the  disk;  so  that  when  the 
animal  is  crawling  their  successive  actions  produce  a  visible 
undulatory  motion  of  that  surface.  The  efiect  of  these  ac- 
tions is  that  different  parts  of  the  plane  on  which  it  moves 
are  laid  hold  of  in  succession,  and  each  corresponding  por- 
tion of  the  animal  is  dragged  along,  so  that  the  body  ad- 
vances by  a  slow  and  uniform  gliding  motion.  The  opera- 
tion of  this  mechanism  may  easily  be  seen  in  a  snail,  by 
making  it  crawl  on  a  pane  of  glass,  and  viewing  the  move- 
ment of  its  disk  from  the  other  side  of  the  glass:  the  regu- 
lar undulations  which  advance  in  the  direction  of  the  mo- 
tion of  the  snail,  but  with  twice  the  velocity,  present  a  cu- 
rious and  interesting  spectacle. 

A  mucilaginous  secretion  generally  exudes  from  the  sur- 
face  of  the   disk,  and   tends  to    increase  considerably  its 


ceived  that  these  threads  are  dried  muscular  fibres;  an  opinion  which  has 
been  adgpted  by  Blahivillc. 


GASTEROPODA.  167 

power  of  adhesion,  both  when  the  animal  is  crawling,  and 
also  when  it  fixes  itself  on  any  surface.  In  the  Patella,  or 
limpet,  this  adhesion  is  greatly  favoured  by  the  conical  form 
of  the  shell,  which  having  a  circular  base,  enables  the  mus- 
cles of  the  disk,  by  their  efforts  to  create  a  vacuum  under- 
neath it,  to  command  the  whole  hydrostatic  pressure  of  the 
superincumbent  water,  as  well  as  of  the  atmosphere  above 
the  water.  Besides  the  muscular  bands  contained  in  the 
substance  of  the  foot,  other  sets  of  fibres  are  provided  for  the 
purpose  of  protruding  or  of  retracting  the  whole  member, 
and  of  moving  it  in  different  directions. 

The  foot  of  the  Buccinmn  xindatuvi,  or  Whelk,  is  capa- 
ble of  great  dilatation  by  means  of  four  tubes,  which  open 
from  the  surface  near  the  gullet,  and  convey  into  it  a  large 
quantity  of  water.  It  may,  by  this  means,  be  distended  to 
a  size  even  greater  than  the  shell  itself;  so  that  the  opening 
which  it  forms  in  the  sand  is  large  enough  to  receive  the 
shell,  when  the  latter  is  drawn  down  by  the  contraction  of 
the  muscles  which  are  attached  to  the  foot.*  The  foot  of 
the  Scyllsea  is  grooved,  for  the  purpose  of  enabling  the  ani- 
mal to  lay  hold  of  the  stems  and  branches  of  marine  plants, 
and  advance  along  them  by  a  gliding  motion. 

The  head  is  generally  furnished  with  tubular  tcntacula, 
which  the  animal  protrudes  for  the  purpose  of  feeling  its 
way  as  it  advances,  and  which  are  quickly  retracted,  \i^  the 
reversion  of  the  tube,  when  they  arc  touched  or  irritated. 
This  mechanism  is  matter  of  familiar  observation  in  the  tcn- 
tacula, or  horns,  of  the  snail  and  of  the  slug,  which  are  ter- 
restrial mollusca  belonjiins  to  this  order.  The  former  of 
these  has  a  turbinated  shell  of  the  ordinary  structure:  the 
latter,  though  extremely  similar  in  its  internal  structure  to 
the  snail,  is  destitute  of  any  external  shell;  but  is  furnished, 
instead  of  it,  with  a  small  internal  plate  of  cartilage,  giving 
support  to  some  of  the  vital  organs. 

•  Osier,  Phil.  Trans,  for  1826,  p.  352. 


16S  THE  MECHANICAL  FUNCTIONS. 


§  4.  Structure  and  Formation  of  the  Shells  of  Mollusca, 

The  structure  and  formation  of  the  shells  of  molluscous 
animals  is  a  subject  of  much  interest  in  comparative  physi- 
ology, as  presenting  many  beautiful  illustrations  of  the  laws 
by  which  the  inorganic  parts  of  the  living  system  are  in- 
creased in  their  dimensions. 

All  shells  are  composed  of  two  portions,  the  one  consist- 
ing of  particles  of  carbonate  of  lime,  the  other  having  the 
character  of  an  animal  substance,  and  corresponding  in  its 
chemical  properties  either  to  albumen  or  to  gelatine.  The 
mode  in  which  these  two  constituent  parts  are  united,  as 
well  as  the  nature  of  the  animal  portion,  differ  much  in  dif- 
ferent kinds  of  shell;  and  it  is  chiefly  in  reference  to  these 
circumstances  that  shells  have  been  divided  into  two  classes, 
namely,  the  memhranous  and  jiorcellaneQUS  shells. 

In  shells  belonging  to  the  first  of  these  classes,  the  carbo- 
nate of  lime  is  united  with  a  membranous  substance  deposit- 
ed in  layers,  which  may  be  separated  from  one  another, 
either  by  mechanical  division  with  a  sharp  instrument,  or 
by  the  slow  actions  of  air,  water,  or  other  decomposing  che- 
mical agents.  The  shells  of  the  limpet,  of  the  oyster,  and 
of  almost  all  the  larger  bivalve  mollusca  which  reside  in  the 
oceaif  are  of  this  kind.  They  are  usually  covered  with  a 
thick  outer  skin,  or  epidermis;  and  their  texture  is  of  a 
coarser  srain  than  that  of  other  shells. 

If  a  shell  of  this  description  be  immersed  in  an  acid  capable 
of  dissolving  carbonate  of  lime,  such  as  the  muriatic  or  ni- 
tric acids  properly  diluted,  at  first  a  brisk  efiervescence  is 
produced,  but  this  soon  slackens,  and  the  carbonate  of  lime 
contained  in  the  shell  is  slowly  dissolved;  the  membranous 
layers  being  left  entire,  and  sufficiently  coherent  to  retain 
the  figure  of  the  shell,  but,  having  lost  the  earthy  material 
which  gave  them  hardness,  they  assume  their  natural  form 
of  soft  and  flexible  plates. 

Many  membranous  shells  exhibit,  on  several  parts  of  their 
internal  surface,  a  glistening,  silvery,  or  iridescent  appear- 


\      ••■^  ">,v   ^ 


STRUCTURE  OF  SHELLS.    '  IQO 

ance.*     This  appearance  is  caused  by  the  peculiar  thinness, 
transparency,  and  regularity  of  arrangement  of  the  outer 
layers  of  the  membrane,  which,  In  conjunction  with  the  par- 
ticles of  carbonate  of  lime,  enter  into  the  formation  of  that 
part  of  the  surface  of  the  shell.     The  surface,  which  has  thus 
acquired  a  pearly  lustre,  was  formerly  believed  to  be  a  pe- 
culiar substance,  and  was  dignified  with  the  appellation  of 
mother  of  pearl,  from  the  notion  that  was  entertained  of  its 
being  the  material  of  which  pearls  are  formed.     It  is  true, 
indeed,  that  pearls  are  actually  composed  of  the  same  mate- 
rials, and  have  the  same  laminated  structure  as  the  mem- 
branous shells;  being  formed  by  very  thin  concentric  plates 
of  membrane  and  carbonate  of  lime,  disposed  alternately, 
^^^^^^106  and  often  surrounding  a  central  body, 

pfi^':    "'-^  or  nucleus:  but  Sir  David  Brewster 

has  satisfactorily  shown  that  the  Iri- 
descent colours  exhibited  by  these 
;|;  surfaces  are  wholly  the  effect  of  the 
parallel  grooves  consequent  upon 
-n^s^^  the  regularity  of  arrangement  in  the 
-'^'^  successive  deposltes  of  shell. t  The 
appearance  of  these  grooves  or  strise 
w^hen  highly  magnified  is  shown  in  Fig.  106.:]:  This  Iride- 
scent property  may  be  communicated  to  shell  lac,  sealing 
wax,  gum  Arabic,  balsam  of  Tolu,  or  fusible  metal,  by  taking 
an  accurate  cast  or  impression  of  the  surface  of  mother  of 
pearl  with  any  one  of  these  substances.§ 

Porcellaneous  shells  have  a  more  uniform  and  compact 
texture  than  those  of  the  former  class.     The  animal  matter 

*  Examples  of  this  nacreous  structure,  as  it  is  termed,  occur  in  the  shells 
of  the  ffalioiis,  or  Sea-ear,  and  of  the  ^iiodon,  or  fresh  water  muscle. 

f  Philosophical  Transactions  for  1814,  p.  397. 

^  See  a  paper  on  this  subject  by  Herschel  in  the  Edinburgh  Philosophical 
Journal,  ii.  114. 

§  When  these  shells  decay  and  fall  to  pieces,  they  separate  into  numerous 
thin  scales  of  a  pearly  lustre.  The  fine  scales  thus  obtained  from  tlxe  Tia- 
cuna,  or  window  oyster,  are  employed  by  the  Cliinese  in  their  water-colour 
drawings  to  produce  the  effect  of  silver.  Some  of  this  powder  hiu>  been 
brought  to  England  and  used  for  this  piu'pose.  Sec  Gra\-,  Phil.  Trans,  for 
1833. 

Vol.  I.  >  22 


■Jj^i^-^  .  1^^  '-^^  ^^i-' 


170  THE  MECHANICAL  FUNCTIONS. 

which  unites  the  carbonate  of  lime  is  less  in  quantity  and 
not  so  evidently  disposed  in  layers;  but  it  is  more  equally 
blended  with  the  earthy  particles,  with  respect  to  which  it  ap- 
pears to  perform  the  office  of  a  cement,  binding  them  strong- 
ly together;  although  it  has  of  itself  but  little  cohesive 
strength.  The  Cyprsea  and  the  Volute  are  examples  of  por- 
cellaneous shells. 

In  shells  of  this  kind  the  carbonate  of  lime  assumes  more 
or  less  of  a  crystalline  arrangement;  the  minute  crystals  be- 
ing sometimes  in  the  form  of  rhombs,  and  sometimes  in  that 
of  prisms.  In  the  former  case  they  are  composed  of  three 
distinct  layers,  as  may  be  seen  by  making  sections  of  any 
of  the  spiral  univalve  shells,  or  simply  by  breaking  them  in 
various  directions.     Each  layer  is  composed  of  very  thin 

plates,  marked  by  oblique  lines,  which 
show  the  direction  of  the  crystalline 
fibres.*  The  direction  of  the  layers 
and  fibres  is  also  rendered  manifest  by 
the  planes  of  cleavage,  when  they  are 
broken  into  fragments.  The  plates  of 
the  outer  and  inner  layers  are  always 
directed  from  the  apex  of  the  cone  to 
its  base,  so  as  to  follow  the  direction  of 
the  spire:  while,  on  the  contrary,  those 
of  the  intermediate  plate  form  concentric  rings  round  the 
cone  parallel  to  its  base.  Thus  the  fibres  of  each  layer  are  at 
rio-ht  andes  to  those  of  the  layer  which  is  contiguous  to  it; 
an  arrangement  admirably  calculated  for  giving  strength  to 
the  shell,  by  opposing  a  considerable  cohesive  resistance  to 
all  forces  tending  to  break  it,  in  whatever  direction  they  may 
be  applied. "^     We  here  find  that  a  principle,  which  has  only 

*  These  lines  are  shown  in  the  diagram,  Fig-.  107,  which  represents  a 
longitudinal  section  of  a  shell  of  this  kind.  A  is  the  outer  layer,  of  which 
the  fibres  pass  obliquely  downwards.  B  is  the  middle  layer,  having-  fibres 
placed  at  right  angles  with  the  former.  C  is  the  third,  or  inner  layer,  the 
fibres  of  which  have  a  direction  similar  to  the  outer  layer.  Within  this  lay- 
er  there  is  frequently  found  a  desposite  of  a  hard,  transparent,  and  apparent- 
ly homogeneous  calcareous  material,  D.  Of  this  latter  substance  I  shall  af- 
terwards have  occasion  to  speak. 


STRUCTURE  OF  SHELLS.  171 

of  late  years  been  recognised  and  applied  to  the  buildins;  of 
ships,  namely,  that  of  the  diagonal  arrangement  of  the  frame- 
work, and  the  oblique  position  of  the  timbers,  is  indentical 
with  that  which  from  the  beginning  of  creation,  has  been 
acted  upon  by  nature  in  the  construction  of  shells. 

When  the  form  of  the  crystals  is  prismatic,  the  fibres  are 
short,  their  direction  is  perpendicular  to  the  surface,  and 
the  prisms  are  generally  hexagonal.  This  structure  is  ob- 
servable in  the  Teredo  giganiea  from  Sumatra,*  and  also 
in  many  bivalves,  such  as  those  belonging  to  the  genera 
Avictda  and  Pinna. 

When  porcellaneous  shells  are  subjected  to  the  solvent 
action  of  acids,  the  animal  matter  in  their  composition  offer- 
ing but  little  resistance,  there  is  a  considerable  and  long  con- 
tinued effervescence.  The  solution  of  the  carbonate  of  lime 
proceeds  rapidly,  in  consequence  of  the  speedy  disintegra- 
tion of  the  animal  substance,  which  is  broken  up,  and  partly 
dissolved.  The  remainder  is  reduced  to  minute  fragments, 
which  subside  in  the  form  of  flakes  or  scales  to  the  bottom 
of  the  fluid.  Poli  has  given  a  minute  and  elaborate  descrip- 
tion of  the  appearances  of  these  fragments  of  membrane, 
when  seen  under  the  microscope.! 

The  difference  between  the  textures  of  these  two  kinds  of 
shell  is  farther  illustrated  by  the  impression  made  upon  them 
by  fire.  Porcellaneous  shells,  when  exposed  to  a  red  heat, 
give  out  neither  smell  nor  smoke;  they  lose,  indeed,  their 
colour,  but  retain  their  figure  unaltered.  JNIembranous  shells, 
on  the  contrary,  emit  a  strong  fetid  odour,  and  become  black; 
after  which  the  plates  separate,  and  the  structure  falls  to 
pieces. 

This  variety  in  the  composition  and  structure  of  different 
kinds  of  shell  is  accompanied  by  corresponding  modilica- 
tions  of  their  mechanical  properties.  The  tougliness  of  the 
fibrous  basis  of  membranous  shells,  imparts  to  them  greater 

*  In  this  shell  the  crystalline  appearance  is  so  perfect,  that  when  some 
frag-mcnts  were  sent  to  England  they  were  mistaken  for  a  minei-ai  production. 
Home;  Lectures,  I.  5o. 

\  See  hb  folio  work  on  the  Testacca  of  the  Two  SiciUcs. 


172  THE  MECHANICAL  FUNCTIONS. 

strength  than  is  possessed  by  the  porcellaneous  shells,  which, 
in  consequence  of  the  tenuity  and  uniform  intermixture  of 
the  animal  cement  with  the  calcareous  particles,  present  a 
harder  and  more  transparent,  but  at  the  same  time  more 
brittle  compound.  It  is  these  qualities,  together  with  their 
smooth  enamelled  surface,  often  beautifully  variegated  with 
brilliant  colours,  and  presenting  altogether  a  close  resem- 
blance to  porcelain,  that  have  procured  them  the  name  they 
*  bear. 

When  the  transparency  and  brittleness  of  these  shells  are 
very  great,  they  have  been  considered  as  forming  another 
class,  and  they  have  been  termed  Vitreous  shells,  from  their 
making  a  nearer  approach  to  glass.  Some  shells  present  in- 
termediate textures  between  the  membranous  and  the  por- 
cellaneous. 

All  those  surfaces  of  the  shell  on  its  outer  side  which  are 
not  in  contact  with  any  part  of  the  animal,  are  originally  co- 
vered with  an  epidermis:*  which,  however,  is  frequently 
rubbed  off  by  friction. 

The  process  employed  by  nature  for  the  formation  and  en- 
largement of  the  shells  of  the  mollusca  was  very  imperfect- 
ly understood  prior  to  the  investigations  of  Reaumur,  who 
may  be  considered  as  having  laid  the  first  solid  foundations 
of  the  theory  of  this  branch  of  comparative  physiology. t 
His  experimental  inquiries  have  fully  established  the  two 
following  general  facts:  first,  that  the  grow^th  of  a  shell  is 
simply  the  result  of  successive  additions  made  to  its  surface, 
and  secondly,  that  the  materials  constituting  each  layer,  so 
added,  are  furnished  by  the  organized  fleshy  substance, 
which  he  termed  the  skin  of  the  animal,  but  which  is  now 
known  by  the  name  of  the  mantle,  and  not  by  any  vessels 
or  other  kind  of  organization  belonging  to  the  shell  itself. 

If  a  portion  of  the  shell  of  a  living  snail,  for  instance,  be 
removed,  which  can  be  done  without  injury  to  the  animal, 
since  it  adheres  to  the  flesh  only  in  one  point,  there  is 

*  This  membrane  has  been  termed  the  Perlostracum. 

f  Memoires  dc  I'Academie  des  Sciences,  1709,  p.  367,  and  1716,  p.  303. 


FORMATION  OF  SHELLS.  173 

formed,  in  the  course  of  twenty-four  hours,  a  fine  pellicle, 
resembling  a  spider's  web,  which  is  extended  across  the  va- 
cant space,  and  constitutes  the  Hrst  stratum  of  the  new  shell. 
This  web,  in  a  few  days,  is  found  to  have  increased  in  thick- 
ness, by  the  addition  of  other  layers  to  its  inner  surface;  and 
this  process  goes  on  until,  in  about  ten  or  twelve  days,  the 
new  portion  of  shell  has  acquired  nearly  the  same  thickness 
as  that  which  it  has  replaced.  Its  situation,  however,  is  not 
exactly  the  sam.e,  for  it  is  beneath  the  level  of  the  adjacent 
parts  of  the  shell.  The  fractured  edges  of  the  latter  remain 
unaltered,  and  have  evidently  no  share  in  the  formation  of 
the  new  shell,  of  which  the  materials  have  been  supplied 
exclusively  by  the  mantle.  This  Reaumur  proved  by  in- 
troducing through  the  aperture  a  piece  of  leather  underneath 
the  broken  edges,  all  round  their  circumference,  so  as  to  lie 
between  the  old  sliell  and  the  mantle:  the  result  was  that  no 
shell  was  formed  on  the  outside  of  the  leather;  while,  on  the 
other  hand,  its  inner  side  was  lined  with  shell. 

The  calcareous  matter  which  exudes  from  the  mantle  in 
this  process  is  at  first  fluid  and  glutinous;  but  it  soon  hardens, 
and  consolidates  into  the  dense  substance  of  the  shell.  The 
particles  of  carbonate  of  lime  are  either  agglutinated  toge- 
ther by  a  liquid  animal  cement,  which  unites  them  into  a 
dense  and  hard  substance,  resembling  porcelain;  or  they  are 
deposited  in  a  bed  of  membranous  texture,  having  already 
the  properties  of  a  solid  and  elastic  plate.  This  explains 
the  laminated  structure  possessed  by  many  shells  of  this 
class,  such  as  that  of  the  oyster,  of  which  the  layers  are  easi- 
ly separable,  being  merely  agglutinated  together  like  the 
component  leaves  of  a  sheet  of  pasteboard. 

It  has  long  been  the  prevailing  opinion  among  naturalists 
that  no  portion  of  a  shell  which  has  been  once  deposited,  and 
has  become  consolidated,  is  capable  of  afterwards  undergoing 
any  alteration  by  the  powers  of  the  animal  that  formed  it. 
Very  conclusive  evidence  has,  in  my  opinion,  been  adduced 
against  the  truth  of  this  theory,  by  ]\lr.  Gray,  in  a  paper 
lately  read  to  the  Royal  Society.  From  a  variety  of  facts,  it 
appears  certain  that  on  some  occasions  the  molluscous  animal 


174  THE  MECHANICAL  FUNCTIONS. 

effects  the  removal  of  large  portions  of  its  shell,  when  they 
interfere  with  its  own  growth,  or  are  otherwise  productive 
of  inconvenience.  We  should  at  the  same  time  regard  these 
cases  in  the  light  of  exceptions  to  the  ordinary  rule,  that  a 
portion  of  shell  once  formed  remains  ever  after  unchanged, 
while  it  continues  to  be  connected  with  the  animal  which 
produced  it.  In  a  general  way,  indeed,  we  may  consider 
the  connexion  between  the  animal  and  the  shell  as  m.echani- 
cal,  rather  than  vital;  and  the  shell  itself  as  an  extraneous 
inorganic  body,  formang  no  part  of  the  living  system:  for 
whatever  share  of  vitality  it  may  have  possessed  at  the 
moment  of  its  deposition,  all  trace  of  that  property  is  soon 
lost.  Accordingly,  we  find  that  the  holes  made  in  shells 
by  parasitic  worm.s  are  never  filled  up,  nor  the  apertures  of 
the  cavities  so  made  covered  over,  unless  the  living  flesh  of 
the  animal  be  wounded;  in  which  case  an  exudation  of  cal- 
careous matter  takes  place,  and  a  pearly  deposite  is  produced. 
The  worn  edges  of  shells,  and  the  fractures,  and  other  ac- 
cidents which  befall  them,  are  never  repaired,  except  as  far 
as  such  repairs  can  be  made  by  the  addition  of  materials  from 
the  secreting  surfaces  of  the  mantle.  It  is  found  that  shells 
may  be  impregnated  with  poisonous  metallic  salts,  such  as 
those  of  copper,  without  any  detriment  to  the  animals  they 
enclose. 

The  power  of  secreting  the  materials  of  shell  does  not 
usually  extend  to  the  whole  of  the  surface  of  the  mantle,  but 
is  generally  confined  to  the  parts  near  the  margin,  composing 
what  is  termed  the  collar.  The  calcareous  substance  is  always 
poured  out  underneath  the  epidermis,*  that  is,  between  this 
outermost  layer  of  integument,  and  the  subjacent  corium, 
which  is  incorporated  with  the  mantle,  and  may  be  regarded 
as  forming  one  and  the  same  organ. t 

*  Mr.  Gray  considers  the  external  membrane  of  the  shell,  or  epidermis,  as 
formed  by  the  outer  edge  of  the  plates  of  animal  substance,  which  have 
scarcely  any  calcareous  matter  in  their  composition,  and  which  are  soldered 
together  into  a  membranous  coat. 

■{"A  secreting  power  is  also,  in  some  instances,  possessed  by  the  foot,  as  is 
exemplified  in  some  of  the  gasteropoda,  where  it  forms  an  operculum,  or 


FORMATION  OF  SHELLS.  175 

The  shape  of  the  shell  depends  altogether  on  tlie  extent 
and  particular  form  and  position  of  the  secreting  organ. 
The  animal,  on  its  exclusion  from  the  egg,  has  already  a 
small  portion  of  shell  formed.  The  simplest  case  is  that 
in  which  this  rudiment  of  shell  is  a  concave  disk.  We 
may  conceive  the  animal,  covered  by  its  mantle,  to  expand 
the  border  of  this  organ,  and  extend  it  beyond  the  edge  of 
the  shell,  where  it  then  forms  a  new  layer  of  shell;  and  this 
new  layer,  being  applied  to  the  inner  or  concave  surface  of 
the  original  shell,  will,  of  course,  extend  a  little  w^ay  beyond 
its  circumference.  The  same  happens  with  the  succeeding 
layers,  each  of  whichbeing  larger  than  the  one  which  has  pre- 
ceded it,  projects  in  a  circle  beyond  it;  and  the  whole  scries 
of  these  conical  layers,  of  increasing  diameters,  forms  a  com- 
pound cone,  of  which  the  outer  surface  exhibits  transverse 
lines,  showing  the  successive  additions  made  to  the  shell  in 
the  progress  of  its  increase.  The  Patella,  or  limpet,  is  an 
example  of  this  form  of  structure. 

But  in  by  far  the  greater  number  of  mollusca  which  inhabit 
univalve  shells,  the  formation  and  deposition  of  the  earthy 
material  does  not  proceed  equally  on  all  sides,  as  happens 
in  the  patella.  If  the  increase  take  place  in  front  only,  that 
is,  in  the  fore  part  of  the  mantle,  the  continual  deflexion 
thence  arising  necessarily  gives  the  shell  a  spiral  form,  the 
coils  being  simply  in  one  plane.  This  is  the  case  in  the 
Planorbis.  (Fig.  105)  the  Spirula,?iv\(\  the  Nautilus.  INIost, 
commonly,  however,  as  in  the  Bucclniiin,  and  Jichatina 
(Fig.  108)  the  deposite  of  shell  takes  place  laterall}^,  and 
more  on  one  side  than  on  the  other;  hence  the  coils  produced 
descend  as  they  advance,  giving  rise  to  a  curve*,  which  is 
continually  changing  its  plane,  being  converted  from  a  spiral 
to  a  helix,  a  term  of  Geometry  borrowed  from  the  Latin 
name  of  common  snail,  which,  as  is  well  known,  has  a  shell 

calcareous  covering-  to  the  mouth  of  the  shell.  Mr.  Gray  also  ascertained  that 
in  the  Cymbia,  and  Olivx,  and  the  AnciUariae^  shell  is  deposited,  and  most 
probably  secreted  by  the  upper  surface  of  tlie  foot,  vvliich  is  very  larg-e,  and 
not  by  the  mantle,  which  is  small,  and  does  not  extend  beyond  the  cdg-e  of 
the  mouth. 


176 


THE  MECHANICAL  FUNCTIONS. 


of  this  form.  Fig.  108,  which  represents  the  shell  of  the 
t/ichaiina  zebra,,  and  of  which  Fig.  109  shows  a  longitudinal 
section,  may  serve  as  an  example  of  a  shell  of  this  kind. 
The  axis  of  revolution  is  termed  the  Columella,  and  the 
turns  of  the  spiral  are  denominated  lohorls.  In  consequence 
of  the  situation  of  the  heart  and  great  blood  vessels  relative- 


ly to  the  shell,  the  left  side  of  the  mantle  Is  more  active  than 
the  risht  side,  so  that  the  lateral  turns  are  made  in  the  con- 
trary  direction,  that  is,  towards  the  right.*  There  are  a 
few  species,  however,  where,  in  consequence  of  the  heart 
being  placed  on  the  right  side,  the  turns  of  the  spiral  are 
made  to  the  left.  Such  shells  have  been  termed  sinistral, 
ov  reversed  shoVi'.  but  this  left-handed  convolution  seldom 
occurs  amons;  the  shells  of  land  or  fresh  water  mollusca. 

It  results  from  this  mode  of  formation  that  the  apex  both 
of  the  simple  and  of  the  spiral  cone  is  the  part  which  was 
formed  the  earliest,  and  which  protected  the  young  animal 
at  the  moment  of  its  exclusion  from  the  egg.  This  portion 
may  generally  be  distinguished  by  its  colour  and  appearance 
from  that  which  is  afterwards  formed.  The  succeeding  turns 
made  by  the  shell  in  the  progress  of  its  growth,  enlarging  in 
diameter  as  they  descend  from  the  apex,  form  by  degrees  a 
wider  base.  During  the  growth  of  the  animal,  as  the  body 
extends  toward  the  mouth  of  the  shell,  its  posterior  end 
often  quits  the  first  turn  of  the  spire,  and  occupies  a  situa- 

*  The  terms  rig-ht  and  left  have  reference  to  the  position  of  the  animal 
when  resting-  on  its  foot;  the  head  being-,  of  course,  in  front.  See  Gray, 
Zool.  Journal,  i.  207. 


FORMATION  OP  SHELLS.  177 

tion  dlfierent  from  that  vvliieh  it  Iiad  originally.  In  these 
cases  the  cavity  at  the  apex  of  the  spire  is  filled  up  with 
solid  calcareous  matter  of  a  hardness  not  inferior  to  that  of 
marble. 

Such  is  the  general  form  of  turbinated  shells.  It  some- 
times happens,  however,  as  in  the  Conns ,  that  the  upper 
surface  of  the  spiral  scarcely  descends  below  the  level  of  the 
original  portion  of  the  shell,  which  in  the  former  disposition 
of  its  parts  would  have  been  the  apex:  while  the  lower  por- 
tions of  the  spiral  turns  shoot  downwards  so  as  to  form  a 
pointed  process;  thus,  the  whole  is  still  a  cone,  but  reversed 
from  the  former,  the  part  last  formed  being  the  outer  surface 
of  the  cone  and  the  circumference  of  the  apparent  base,  or 
flat  surface,  of  which  the  central  part  is  the  one  first  formed. 

Various  causes  may  occur  to  disturb  the  regularity  of  the 
process  of  deposition,  by  which  the  shell  is  enlarged  in  its 
dimensions:  at  one  time  accelerating,  and  at  another  retard- 
ing, or  totally  arresting  its  growth.  These  irregularities  are 
productive  of  corresponding  inequalities  in  the  surface  of 
the  shell,  such  as  transverse  lines,  or  striae.  Whenever  an 
exuberance  of  materials  has  led  to  a  sudden  expansion  of 
growth,  which  has  again  soon  subsided,  a  projecting  ridge  is 
produced  in  the  direction  of  the  margin  of  the  mantle  at  the 
time  this  happens.  This  change  generally  recurs  at  regular 
periods,  so  that  these  ridges,  or  ribs,  as  they  are  often  called, 
succeed  one  another  at  equal  distances  along  the  course  of 
the  spiral  turns. 

It  not  unfrequently  happens,  that  at  diiTcrent  periods,  a 
sudden  development  takes  place  in  particular  parts  of  the 
mantle,  which  become  in  consequence  rapidly  enlarged, 
shooting  out  into  long  slender  processes.  Every  part  of  the 
surface  of  these  processes  has  the  power  of  secreting  and 
forming  shell,  so  that  the  portion  of  shell  they  construct, 
being  consolidated  around  each  fleshy  process,  must  neces- 
sarily have  at  first  the  shape  of  a  tube  closed  at  the  extre- 
mity. As  fresh  dcposites  are  made  by  the  secreting  surface, 
which  are  in  the  interior  of  the  tube,  the  internal  space  is 

Vol.  I.  23 


178 


THE  MECHANICAL  TUNCTIONS. 


gradually  filled  up  by  these  deposites;  the  process  of  the 
mantle  retiring  to  make  way  for  their  advance  towards  the 
axis  of  the  tube.  In  the  course  of  time,  every  part  of  the 
cavity  is  obliterated,  the  process  of  the  shell  becoming  en- 
tirely solid.  Such  is  the  origin  of  the  many  curious  pro- 
jecting cones  or  spines  which  several  shells  exhibit,  and 
which  have  arisen  periodically  during  their  growth  from 
their  outer  surface.  In  the  Murex  these  processes  are  of- 
ten exceedingly  numerous,  and  occur  at  regular  intervals, 
frequently  shooting  out  into  various  anomalous  forms.  In 
many  shells  of  the  genus  Strombus  these  spines  are  of  great 
length,  and  are  arranged  round  the  circumference  of  the 
base,  being  at  first  tubular,  and  afterwards  solid,  according 
to  the  period  of  growth.  This  is  exemplified  in  the  Piero- 
cera  scorpio  (Lamarck)  of  which  Fig.  110  shows  the  early, 
and  Fig.  Ill  the  later  period  of  growth. 


A  limit  has  been  assigned  by  nature  to  the  growth  of 
molluscous  animals,  and  to  the  shells  which  they  form:  and 
there  is  a  certain  epocli  of  their  existence,  when  consider- 
able changes  take  place  in  the  disposition  of  the  mantle,  and 
in  its  powers  of  secretion.  Often  w^e  find  it  suddenly  ex- 
panding into  a  broad  surface,  adding  to  the  shell  what  may 
be  termed  a  large  lip.  Sometimes  no  sooner  has  this  been 
accomplished  than  the  same  part  again  shrinks,  and  the 
mantle  retires  a  little  way  within  the  shell,  still  continuing 
to  deposite  calcareous  layers,  which  give  greater  thickness 
to  the  adjacent  part  of  the  shell:  and  at  the  same  time  nar- 


FORMATION  OF  SHELLS.  179 

row  its  aperture,  and  materially  alter  its  general  shape  and 
aspect.  Thus  it  happens  that  the  shells  of  the  young;  and  of 
the  old  individuals  of  the  same  species  are  very  different, 
and  would  not  be  recognised  as  belonging  to  the  same  tribe 
of  mollusca.  This  is  remarkably  the  case  with  the  shell  of 
the  Cypvcca,  or  Cowrie,  which  in  the  early  stage  of  its 
growth,  (Fig,  112)  has  the  ordinary  form  of  an  oblong  tur- 
binated shell:  but  from  the  process  just  described  taking 
place  at  a  certain  period,  the  mouth  of  the  shell  (as  shown 
in  Fig.  113,)  becomes  exceedingly  narrow,  and  the  edges  of 
the  aperture  are  marked  by  indentations,  moulded  on  cor- 
responding processes  of  the  mantle.*  But  in  this  instance 
the  change  does  not  stop  here;  for  both  edges  of  the  mantle 
next  take  a  wider  expansion,  turning  over  the  outer  surface  of 
the  shell,  and  passing  on  till  they  meet  at  the  upper  convex 
part,  or  back  of  the  shell,  forming  what  has  been  termed  the 
dorsal  line.  They  dcposite,  as  they  proceed,  a  dense  and  high- 
ly polished  porcellaneous  shell,  beautifully  variegated  with 
coloured  spots,  which  correspond  exactly  with  tlie  coloured 
1X4  parts  of  the   mantle   that  deposites 

them.  This  new  plate  of  shell  com- 
pletely envelops  the  original  shell, 
giving  it  a  new  covering,  and  dis- 
suisins:  its  former  character.  A 
transverse  section,  (Fig.  114,)  at  once 
shows  the  real  steps  by  which  these 
changes  have  taken  placet 
Changes  equally  remarkable  are  observed  to  occur  in  the 
interior  of  the  shell  at  different  stages  of  its  growth.     On 

•  Similar  chang-es  occur  in  the  shells  of  the  Ovula  (spindles,)  Erato  (tear- 
shells,)  and  Margmella^  (dates.)   Gray,  Phil.  Trans,  for  1833. 

f  According  to  Brug-uiere,  there  is  reason  to  believe  that  the  animal  of  the 
Cy/Jra^a  after  having  completed  its  shell,  in  the  manner  above  described,  still 
continuing-  to  grow,  and  being  incommoded  for  want  of  space,  quits  its  shell 
altogether,  and  sets  about  forming  a  new  one,  better  suited  to  its  enlarged 
dimensions.  It  is  stated  also  that  the  same  individual  is  even  cai)able  of 
forming  in  succession  several  shells.  Blalnvillc,  however,  considers  it  im- 
possible that  the  living  animal  can  ever  quit  its  shell.     Malacologle,  p.  94. 


ISO  THE  MECHANICAL  FUNCTIONS. 

the  inner  surface  of  the  Mitra,  the  Volute,  and  other 
shells  of  a  similar  kind,  there  is  deposited  a  layer  of  a 
hard  semi-transparent  calcareous  material,  having  a  vitreous 
appearance.*  The  thickness  of  the  layer,  which  thus  lines 
the  cavity  of  the  shell,  is  greater  as  it  approaches  the  apex; 
and  where  the  spire  is  much  elongated,  or  turrited,  as  it  is 
called,!  this  deposition  entirely  fills  the  upper  part,  which, 
in  the  early  condition  of  the  shell,  was  a  hollow  space  with 
thin  sides.  The  purpose  answered  by  this  deposite  is  evident- 
ly to  give  solidity  and  strength  to  a  part  which  by  remain- 
ing in  its  original  state  would  have  been  extremely  liable  to 
be  broken  off  by  the  action  of  the  sea. 

In  other  cases  a  different  expedient  is  adopted.  The 
animal,  instead  of  fortifying  the  interior  of  the  apex  by  a 
lining  of  hard  shell,  suddenly  withdraws  its  body  from  that 
part,  and  builds  a  new  wall  or  partition  across  the  cavity, 
so  as  to  protect  the  surface  thus  withdrawn.  That  portion 
of  the  shell  which  is  thus  abandoned,  being  very  thin  and 
brittle,  and  having  no  support  internally,  soon  breaks  off, 
leaving  what  is  termed  a  decollated  shell;  examples  of  this 
occur  in  the  Cerithiurn,  decollatum,  the  Bulimus  decolla- 
tus,  &c.  The  young  of  the  genus  Magilus  has  a  very  thin 
shell  of  a  crystalline  texture;  but  when  it  has  attained  its 
full  size,  and  has  formed  for  itself  a  lodgement  in  a  coral, 
it  fills  up  the  cavity  of  the  shell  with  a  glassy  deposite, 
leaving  only  a  small  conical  space  for  its  body;  and  it  con- 
tinues to  accumulate  layers  of  this  material,  so  as  to  main- 
tain its  body  *at  a  level  with  the  top  of  the  coral  to  which  it 
is  attached,  until  the  original  shell  is  quite  buried  in  this  vi- 
treous substance. 

The  forms  of  the  Cone  and  Olive  shells  are  such  as 
to  allow  but  a  small  space  for  the  convolutions  of  the 
body  of  the  animal,  which  accordingly  becomes,  in  the 
progress  of  its  enlargement,  excessively  cramped.  In  or- 
der to  obtain  more  space,  and  at  the  same  time  lighten  the 

*  This  is  the  substance  represented  at  d,  Fig.  107,  p.  170. 

\  As  iji  the  genera  TurritdlUy  Tercbra,  Vcrithium,  and  Fa^ciolaria. 


FORIMATION  OF  SHELLS. 


181 


shell,  the  whole  of  the  two  exterior  layers  of  the  inner 
whorls  of  the  shell  are  removed,  leaving  only  the  interior 
layer,  which  is  consequently  very  thin  when  compared  with 
the  other  whorl,  that  envelops  the  whole,  and  which,  re- 
taining its  original  thickness,  is  of  suilicicnt  strength  to  give 
full  protection  to  the  animal.  That  this  change  has  actually 
been  effected  is  very  distinctly  seen  in  the  Conus  (Fig.  115) 
by  examining  a  vertical  section  of  that  shell,  as  is  represented 
in  Fig.  116.     All  the  inner  partitions  of  the  cavity  thus  laid 


115 


117 


open  are  found  to  be  extremely  thin  and  transparent,  and 
to  consist  only  of  the  innermost  lamina  of  the  original  shell; 
as  will  appear  on  tracing  them  up  to  that  outer  portion 
of  the  section  b  b,  which  lies  on  each  side  of  the  proper 
apex  of  the  shell,  and  which  forms  the  apparent  base.  The 
lines  on  this  part  of  the  section  indicate  the  thickness  which 
each  successive  whorl  had  originally,  and  when  it  was  itself 
the  outermost  whorl.  The  section  also  shows  the  vitreous  de- 
posite  which  lines  the  upper  parts  of  the  cavity,  and  which 
completely  fills  up  the  smaller  turns  of  the  spire,  near  the 
apex.* 

There  are,  indeed,  instances  among  shells  of  the  total  re- 
moval of  the  interior  whorls.  This  is  found  to  occur  in  that 
of  the  genus  */luricida,  which  are  molluscous  animals,  res- 


*  Fig".  117,  which  is  a  transveree  section  of  the  same  shell,  shows  the  spi- 
ral convoUitions,  and  the  comparative  thinness  of  the  inner  portions.  It  also 
forms  a  striking  contrast  with  a  similar  section  of  the  Cyprxa,  Fig  11 -I- 


182  THE  MECHANICAL  FUNCTIONS.  . 

piring  by  means  of  pulmonary  organs.     In  the  young  shell 
of  this  tribe,  the  partitions  which  separate  the  cavities  of  the 
whorls  are  incomplete,  and  twine  parallel  to  each  other;  but 
they  wholly  disappear  as  the  animal  approaches  to  maturity. 
In  other  cases,  the  animal  is  found  to  remove  exterior  por- 
tions of  shell  formerly  deposited,  when  they  lie  in  the  way 
of  its  farther  growth,  and  when  the  mouth  of  the  spire  is 
advancing  over  the  irregular  surface  of  the  preceding  whorls. 
Thus  we  often  find  that  the  ridges,  ribs,  or  processes  which 
had  been  deposited  on  the  surface  of  the  shells  of  the  Tri- 
ton, Miirex,  &c.  are  removed  to  make  way  for  the  succeed- 
ing turn  of  the  spire.     In  other  cases,  however,  no  such 
power  of  destroying  portions  of  shell  previously  deposited 
seems  to  exist;  and  each  successive  whorl  is  moulded  upon 
the  one  which  it  covers. 

It  may  also  be  observed,  that  some  mollusca  have  the 
means  of  excavating  the  shells  of  other  animals  on  which 
they  may  choose  to  fix,  for  the  purpose  of  forming  a  conve- 
nient lodgement  for  themselves.  The  Fileopsis  (or  fool's 
cap)  has  this  faculty  in  a  remarkable  degree;  and  it  is  also 
met  with  occasionally  in  SlphonarHce  and  Patellar.  The 
common  Patella^  or  limpet  of  our  own  coasts,  often,  indeed, 
forms  for  itself,  by  some  unknown  process,  a  deep  cavity 
out  of  a  calcareous  rock. 

When  the  animal  which  inhabits  a  spiral  shell  retires 
within  it,  the  only  part  of  its  body  that  is  exposed  to  injury 
is  that  which  is  situated  at  the  mouth  of  the  shell.  With 
a  view  to  its  protection,  it  constructs,  in  many  instances,  a 
separate  plate  of  shell,  adapted  to  the  aperture,  and  denomi- 
nated an  Operculum.  This  piece  is  constructed  by  a  pro- 
cess similar  to  that  by  which  the  rest  of  the  shell  is  formed; 
that  is,  by  the  deposition  of  successive  layers  on  the  inter- 
nal surface,  sometimes  in  an  annular,  and  sometimes  in  a  spi- 
ral form.  If  an  operculum  were  to  be  constructed  of  a  consi- 
derable size,  and  were  connected  to  the  shell  itself  by  a  re- 
gular hinge,  it  would  be  entitled  to  be  considered  as  a  dis- 
tinct valve.  Here,  therefore,  we  perceive,  as  was  remarked 
by  Adanson,  a  connecting  link  between  the  univalve  and 


FORMATION  OP  SHELLS. 


183 


118 


the  bivalve  tcstacca.  A  Clausiinn  is  another  kind  of  co- 
vering, serving  also  for  protection,  and  consisting  of  a  thin 
spiral  plate  of  shell,  attached  to  the  columella  by  an  elastic 
spring,  by  which  the  plate  is  retracted  when  the  animal  re- 
tires into  its  shell.  It  thus  corresponds  exactly  in  its  office 
to  a  door,  opening  and  closing  the  entrance  as  occasion  re- 
quires. An  Epiphragma  is  a  partition 
of  a  membranous  or  calcareous  nature, 
constructed  merely  for  temporary  use. 
It  is  employed  for  closing  the  aperture 
of  the  shell  during  certain  periods  only, 
such  as  the  winter  season,  or  a  long  con- 
tinued drought.  Fig.  118  exhibits  the 
lines  which  appear  on  the  inner  side  of 
the  epiphragma,  of  the  Helix pomatia,  or 
garden  snail,  and  which  indicates  the  succession  of  depositcs 
by  which  it  has  been  formed. 

It  is  remarkable  in  how  short  a  time  this  species  of  Helix 
will  construct  this  covering,  when  circumstances  occur  to 
urge  its  completion.     On  the  approach  of  winter,  the  animal 
prepares  itself  for  passing  that  season  in  a  state  of  torpidity, 
first,  by  choosing  a  safe  retreat;  and  next  by  retiring  com- 
pletely within  its  shell,  and  then  barricading  its  entrance  by 
constructing  the  epiphragma  just  described,  and  of  which 
the  outer  surfiice  is  represented  in  Fig.  119.     Having  formed 
this  first  barrier,  the  animal  afterwards  constructs  a  second, 
of  a  membranous  nature,  situated  more  internally  than  the 
first,  and  at  a  little  distance  from  it.     If  at  any  other  season, 
while  the  snail  is  in  full  vigour,  the  experiment  be  made  of 
surrounding  it  with  a  freezing  mixture,  it  will  immediately 
set  about  constructing  a  covering  for  its  protection  against 
the  cold;  and  it  works  with  such  diligence,  that  in  the  course 
of  an  hour  or  two,  it  will  have  completed  its  task,  and  formed 
an  entire  epiphragma.^     When  the  genial  warmth  of  return- 
ing spring  has  penetrated  into  the  abode  of  the  snail,  the 
animal  prepares  for  emerging  from  ils  prison,  by  secreting 
a  small  quantity  of  a  mucous  fluid,  which  loosens  the  adhc- 


*  Gray,  Zoolog-lcul  Journal,  i.  214. 


184  THE  MECHANICAL  FUNCTIONS. 

sioii  that  had  taken  place  between  the  epiphragma  and  the 
sides  of  the  aperture;  and  the  former  is,  by  the  pressure  of 
the  foot  of  the  snail,  thrown  off.  The  whole  of  this  process 
of  construction  has  to  be  renewed,  on  every  occasion  when 
another  covering  is  required.* 

One  great  use  of  these  coverings  is  to  prevent  evaporation 
from  the  surface  of  the  body  of  the  animal.  It  is  thus  that 
Snails,  Bulimi,  &c.  may  be  preserved  for  months,  and  even 
years  in  a  torpid,  but  living  state,  ready  to  be  restored  to 
the  active  functions  of  life,  when  sufficient  water  is  supplied.t 

The  enlargement  of  bivalve  shells  is  conducted  on  the  same 
principles  as  that  of  univalves;  the  augnlentation  of  bulk 
taking  place  principally  at  the  outer  margin  of  each  valve, 
and  corresponding  with  the  growth  of  the  included  animal. 
The  order  of  succession  in  which  the  layers  are  deposited  is 
clearly  indicated  by  the  lines  on  the  surface,  which  frequent- 
ly appear  of  different  hues  from  the  addition  of  colouring 
particles  secreted  at  particular  periods  by  the  mantle. 

The  shells  of  Oysters  and  other  acephalous  mollusca  which 
adhere  to  rocks,  are  often  moulded,  during  their  growth,  to 
the  surfaces  to  which  they  are  applied.  The  mantle,  being 
exceedingly  flexible,  accommodates  itself  to  all  the  inequali- 
ties it  meets  with,  and  depositing  each  successive  layer  of 
shell  equally  on  every  part,  the  figure  of  the  surface  is  as- 
sumed, not  only  by  the  valve  in  contact  with  it,  but  also  by 
the  other  valve,  which  is  formed  by  the  opposite  surface  of 
the  mantle,J  and  which  during  its  formation  was  immediate- 
ly superposed  on  the  thin  edge  of  the  other  valve,  while  it 
was  deflected  by  the  irregular  surface  on  which  it  grew. 
As  the  enlargement  of  the  shell  proceeds,  it  was  necessary 
that  the  muscle,  which  closes  the  valves,  and  is  attached  to 

•  An  epiphragma  differs  from  true  shells  in  having  no  adhesion  in  any  part 
to  the  animal  which  formed  it. 

■f  A  remarkable  instance  of  this  apparent  reviviscence  of  snails,  which  had 
lain  for  many  years  i)i  a  dormant  state  in  a  cabinet  of  shells,  and  whicli 
crawled  out  on  being  accidentally  put  into  warm  water,  is  recorded  in  the 
Philosophical  Transactions  for  1774,  p.  432. 

4:  Defrance,  Annales  des  Sciences  Naturellcs,  ii.  16. 


I\rOLLUSCA  PTEROrODA.  lS5 

their  inner  surface,  should  he  gradually  removed  to  a  great- 
er distance  from  the  hinge,  so  that  it  may  preserve  its  rela- 
tive situation  with  regard  to  the  whole  shell,  and  retain  un- 
diminished its  power  of  acting  upon  the  valves.  For  this 
purpose  its  adhesions  are  gradually  transferred,  by  some  un- 
known process,  along  the  surface  of  the  valves;  and  the  pro- 
gress of  the  removal  may  generally  be  distinctly  traced  by 
the  marks  which  are  left  in  the  shell  at  the  places  before  oc- 
cupied by  the  attachments  of  the  muscular  fibres.  The  same 
process  takes  place  when  there  are  two  or  three  muscles  in- 
stead of  one. 

A  few  genera  of  Mollusca,  such  as  the  Phohts,  have,  in 
addition  to  the  two  principal  valves,  small  supplementary 
pieces  of  shell.  They  have  been  accordingly  comprised  in 
the  order  of  Midtivalves.  which  also  comprehends  Cuvier's 
order  of  Cirrhopoda,  including  the  several  kinds  of  Barna- 
cles, (the  genus  Lepas  of  Linnaeus,)  which  are  furnished 
wnth  a  great  number  of  jointed  fdaments,  or  c/rr/iz,  and  form 
an  intermediate  link  of  connexion  between  the  Mollusca 
and  the  Jlrticidata.  But  the  limits  of  this  treatise  will  not 
allow  me  to  dwell  on  the  endless  diversities  of  structure 
which  this  subject  presents. 

§  5.  Pteropoda. 

In  the  Mollusca  belonging  to  the  two  orders  which  have 
now  passed  under  our  review,  namely,  the  Jlcephala  and 
Gasteropoda,  the  mantle,  while  it  folds  over  the  principal 
viscera  of  the  body,  leaves  apertures  for  the  admission  of 
water  to  the  gills,  or  organs  of  respiration.  But  there  exist 
a  few  genera  having  the  sac  formed  by  the  mantle  closed 
on  every  side;  a  structure  which  renders  it  necessary  to 
adopt  a  different  arrangement  with  regard  to  the  gills,  and 
to  place  them  externally,  and  we  then  find  them  spreading 
out  like  a  pair  of  wings,  on  each  side  of  the  neck.  Since 
this  general  closing  of  the  mantle  precludes,  also,  the  for- 
mation of  any  organ  of  progressive  motion  corresponding 
to  a  foot,  advantage  is  taken  of  the  projection  of  the  gills  to 
Vol.  I.  24 


186  THE  MECHANICAt  FUNCTIONS. 

employ  them  as  oars  for  the  purpose  of  enabling  the  animal 
to  swim  through  the  water.  ' 

Mollusca  of  this  description  are  found  in  great  abundance 
in  the  colder  regions  of  the  ocean  surrounding  both  the 
Y2Q  north  and  south  poles;  and  other  species 

are  also  met  with,  though  in  smaller  num- 
bers in  the  tropical  seas.  The  Clio  borea- 
lis,  of  which  Fig.  120  is  a  representation, 
is  the  most  perfect  specimen  of  this  form 
of  construction.  It  swarms  in  the  Arctic 
seaSj  and  constitutes  the  princij^al  food  of 
the  whale.  The  position  of  its  gills,  which 
perform  the  office  of  oars  or  feet,  at  the 
same  time  that  they  resemble  in  their  shape  and  action  the 
wings  of  an  insect,  are  characters  which  have  suggested  the 
title  of  Pterojjoda,  given  by  Cuvier  to  this  order  of  Mol- 
lusca. 

§  6.    Cej)halopodat 

Following  the  progress  of  organic  development,  we  now 
arrive  at  a  highly  interesting  family  of  Mollusca,  denomi- 
nated the  Cephalopoda,  and  distinguished  above  all  the  pre- 
ceding orders  by  being  endowed  with  a  much  more  elabo- 
rate organization,  and  a  far  wider  range  of  faculties.  The 
Cephalopoda  have  been  so  named  from  the  position  of  cer- 
tain organs  of  progressive  motion,  which  are  situated  on  the 
head,  and  like  the  tentacula  of  the  Polypus,  surround  the 
opening  of  the  mouth.  (See  Fig.  121.)  These  feet,  or  arms, 
or  tentacula,  if  we  choose  so  to  call  them,  are  long,  slender, 
and  flexible  processes  exceedingly  irritable,  and  contractile, 
in  every  part,  and  provided  with  numerous  muscles,  which 
are  capable  of  moving  and  twisting  them  in  all  directions  with 
extraordinary  quickness  and  precision.  They  are  thus  ca- 
pable of  being  employed  as  instruments,  not  only  of  progres- 
sive motion,  but  also  of  preliension.  For  this  latter  pur- 
pose they  are  in  many  species  peculiarly  well  adapted,  be- 
cause being  perfectly  flexible  as  well  as  highly  muscular, 


MOLLUSCA  CEPHALOPODA. 


187 


they  twine  with  ease  round  an  object  of  any  shape,  and  grasp 
it  with  prodigious  force.  In  addition  to  these  properties 
they  derive  a  remarkable  power  of  adhesion  to  the  surfaces 


of  bodies  from  their  being  furnished  with  numerous  suckers 
all  along  their  inner  sides.  Each  of  these  suckers,  as  shown 
separately  in  Fig.  122,  is  usually  supported  on  a  narrow 
neck,  or  pedicle,  and  strengthened  at  its  circumference  by 
a  ring  of  cartilage.  Their  internal  mechanism  is  more  ar- 
tificial than  the  simple  construction  already  described,  (p. 


106;)  for  when  the  surface  of  the  disk  is  fully  expanded,  as 
shown  in  Fig.  123*  b,  we  find  that  it  is  formed  of  a  great 
number  of  long  slender  pieces,  resenibling  teeth  closely  set 
together,  and  extending  from  the  inner  margin  of  the  cartila- 
^^,  ginous  ring  in  the  form  of  converging  radii,  to  within  a  short 
'distance  of  the  centre,  where  they  leave  a  circular  aperture. 
In  the  flattened  state  of  the  sucker,  this  aperture  is  filled  by 
the  projecting  part  of  a  softer  substance,  which  forms  an  in- 
terior portion,  capable  of  being  detached  from  the  flat  cir- 
cle of  teeth,  when  the  sucker  is  in  action,  and  of  leaving  an 


188  THE  MECHANICAL  FUNCTIONS. 

intervening  cavity.  The  form  of  this  cavity  is  exhibited  in 
Fig.  c,  which  represents  a  perpendicular  section  of  the  whole 
organ,  and  where  the  central  portion  or  principal  mass  of  the 
sucker  is  drawn  away  from  the  circular  disk,  the  inner  mar- 
gin of  which  appears  like  a  row  of  teeth.  It  is  evident  that 
by  this  mechanism,  which  combines  the  properties  of  an  ac- 
curate valve,  with  an  extensive  cavity  for  producing  rare- 
faction, or  the  tendency  to  a  vacuum,  the  power  of  adhesion 
is  considerably  augmented.* 

So  great  is  the  force  with  which  the  tentacula  of  the  cut- 
tle-fish adhere  to  bodies  by  means  of  this  apparatus,  that 
while  their  muscular  fibres  continue  contracted,  it  is  easier 
to  tear  away  the  substance  of  the  limb,  than  to  release  it  from 
its  attachments.  Even  in  the  dead  animal  I  have  found  that 
the  suckers  retain  considerable  power  of  adhesion  to  any 
smooth  surface  to  which  they  may  be  applied. 

Our  attention  must  first  be  directed  to  the  remarkable  fa- 
mily of  Sepias,  which  comprehends  three  principal  genera, 
namely  the  Octopus,  the  Loligo,  or  Calamary,  (depicted  in 
Fig.  121,)  and  the  common  Sepia,  or  Cuttle-fish.  The  first 
of  these,  the  Octopus,  which  was  the  animal  denominated 
Polypus  by  Aristotle,  has  eight  arms  of  equal  length,  and 
contains  in  its  interior  two  very  small  rudimental  shells, 
formed  by  the  inner  surface  of  the  mantle.  This  shell  be- 
comes much  more  distinct  in  the  Loligo,  where  it  is  carti- 
laginous, and  shaped  like  the  blade  of  a  sword.  (Fig.  123.) 
The  internal  shell  of  the  common  Sepia  is  large  and  broad, 
and  composed  wholly  of  carbonate  of  lime:  it  is  well  known 
by  the  name  of  the  ciittlc-Jish  bone.  Its  structure  is  ex- 
tremely curious;  and  deserves  particular  attention,  as  estab- 
lishing the  universality  of  the  principles  which  regulate  the 
formation  of  shells,  whether  internal  or  external,  and  from 

*  The  description  I  have  here  given  is  the  result  of  my  own  examination 
of  a  large  Octopus,  which  I  had  lately  an  opportunity  of  dissecting:  and  the 
annexed  ^gures  123, »  a,  e,  c,  are  copied  from  drawings  I  made  on  that  oc- 
casion. A  represents  the  sucker  in  its  usual  form  when  not  in  action:  b 
shows  the  sucking  surface  fully  expanded:  and  c  is  a  section  of  the  whole, 
which  had  become  somewhat  flattened  by  the  operation  of  dividing  it. 


♦  .       MOLLUSC  A  CEPHALOPODA.  189 

which  structures  dlfTering  much  in  their  outward  appearance 
may  result.  It  is  composed  of  an  immense  number  of  tliln 
calcareous  plates,  arranged  parallel  to  one  another  and  con- 
nected by  thousands  of  minute  hollow  pillars  of  the  same 
calcareous  material,  passing  perpendicularly  between  the  ad- 
jacent surfaces.  This  shell  is  not  adherent  to  any  internal 
part  of  the  animal  which  has  produced  it;  but  is  enclosed  in 
a  capsule,  and  appears  like  a  foreign  body  impacted  in  the 
midst  of  organs,  with  which,  at  first  sight,  it  would  appear 
to  have  no  relation.  It,  no  doubt,  is  of  use  in  giving  me- 
chanical support  to  the  soft  substance  of  the  body,  and  espe- 
cially to  the  surrounding  muscular  flesh;  and  thus  probably 
contributes  to  the  high  energy  which  the  animal  displays 
in  all  its  movements.  It  has  been  regarded  as  an  internal 
skeleton;  but  it  certainly  has  no  pretensions  to  such  a  desig- 
nation; for,  although  enveloped  by  the  mantle,  it  is  still 
formed  by  that  organ;  and  the  material  of  which  it  is  com- 
posed is  still  carbonate  of  lime.  On  both  these  accounts  it 
must  be  considered  as  a  true  shell,  and  classed  among  the 
productions  of  the  integuments.  It  differs,  indeed,  alto- 
gether from  bony  structures,  which  are  composed  of  a  dif- 
ferent kind  of  material,  and  formed  on  principles  of  growth 
totally  dissimilar.* 

Besides  tentacula,  the  Sepia  is  also  furnished  with  a  pair 
of  fleshy  fins,  extending  along  the  two  sides  of  the  body. 
The  Loligo  has  similar  organs  of  a  smaller  size,  and  situated 
only  at  the  extremity  of  the  body  which  is  opposite  to  the 
head.  They  have  been  regarded  as  the  rudiments  of  true 
fms,  which  are  organs,  developed  in  fishes,  and  which  are 
supported  by  slender  bones,  called  rays;  but  no  structure  of 
this  kind  exists  in  the  fins  of  the  Cephalopoda. 

In  swimming,  the  organs  principally  employed  by  cuttle- 

*  Some  analogies  have,  indeed,  been  attempted  to  be  traced  between  the 
cartilag-lnous  himina  of  the  Loligo,  and  the  spinal  column  of  the  lowest  order 
of  cartilaginous  fishes:  these  I  shall  have  occasion  to  point  out  in  the  sequel. 
Solid  cartilaginous  structures  also  exist  in  the  interior  of  the  body  of  the  ce- 
phalopoda, which  are  considered  by  some  natiu-idists  as  indicating  an  approach 
to  the  formation  of  an  internal  skeleton,  analogous  to  that  of  vcrtcbratcd  ani- 
mals. 


190  THE  MECHANICAL    FUNCTIONS. 

fish  for  giving  an  effective  impulse  to  the  water,  are  the  ten- 
tacula.  These  they  employ  as  oars,  striking  with  them  from 
behind  forwards,  so  that  their  effect  is  to  propel  the  hinder 
part  of  the  body,  which  is  thus  made  to  advance  foremost, 
the  head  following  in  the  rear.  They  also  use  these  organs 
as  feet  for  moving  along  the  bottom  of  the  sea.  In  their  pro- 
gress, under  these  circumstances,  the  head  is  always  turned 
downwards,  and  the  body  upwards,  so  that  the  animal  may 
be  considered  as  literally  walking  upon  its  head.  The  ne- 
cessity of  this  position  for  the  feet  arises  probably  from  the 
close  investment  of  the  mantle  over  the  body;  for  although 
the  mantle  leaves  an  aperture  in  the  neck  for  the  entrance  of 
water  to  the  respiratory  organs,  yet,  in  other  respects,  it 
forms  a  sac,  closed  in  every  part,  except  where  the  head, 
neck,  and  accompanying  tentacula  protrude. 

In  the  Calamary,  as  well  as  in  the  common  Sepia,  two  of 
the  arms  are  much  longer  than  the  rest,  and  terminate  in  a 
thick  cylindrical  portion  covered  with  numerous  suckers, 
which  may  not  unaptly  be  compared  to  a  hand.  These  pro- 
cesses are  employed  by  cuttle-fish  as  anchors  for  the  purpose 
of  fixing  themselves  firmly  to  rocks,  during  violent  agitations 
of  the  sea;  and  accordingly  we  find  that  it  is  only  the  ex- 
tremities of  these  long  tentacula  that  are  provided  with 
suckers,  while  the  short  ones  have  them  along  their  whole 
length. 

The  other  genera  of  cephalopodous  Mollusca  are,  like  the 
Sepise,  provided  with  tentacula  attached  to  the  head.  They 
comprehend  animals  differing  exceedingly  in  their  size: 
some  being  very  large,  but  a  great  number  very  minute,  and 
even  microscopic*  The  shells  of  these  animals  are  often 
found  to  contain  partitions  dividing  them  into  a  number  of 
chambers;  hence  they  have  been  termed  ccnneraied,  or  mul- 
tilocidar^  ov polythalamous  shells.  The  Spirula  (Fig.  124) 
is  a  shell  of  this  description,  of  which  the  cellular  structure 
and  numerous  partitions  are  rendered  visible  by  making  a 

*  A  particular  account  has  been  given  of  the  shells  of  these  microscopic 
cephalopoda  by  M.  D'Orbig-ny,  in  the  Annales  des  Sciences  Naturelles;  vii. 
96. 


MOLLUSCA  CEPHALOPODA. 


irn 


section  through  it:  (Fig.  125.)  Some,  however,  as  the  Jlr- 
gonaut,  or  Paper  Nautilus,  have  shells  undivided  hy  par- 
titions; and  are  accordingly  termed  unilocular  or  mono- 
thalamous.  The  shell  of  the  Argonaut  is  exceedingly  thin, 
iiand  almost  pellucid,  probahly  for  the  sake  of  lightness,  for 
it  is  intended  to  be  used  as  a  boat.  For  the  purpose  of  ena- 
bling the  animal  to  avail  itself  of  the  impulses  of  the  air, 
while  it  is  thus  floating  on  the  waters,  nature  has  furnished 


126 


it  with  a  thin  membrane,  which  she  has  attached  to  two  of 
the  tentacula,  so  that  it  can  be  spread  out  like  a  sail  to  catch 
the  light  winds  which  waft  the  animal  forw^ards  on  its  course. 
While  its  diminutive  bark  is  thus  scudding  on  the  surface  of 
the  deep,  the  assiduous  navigator  does  not  neglect  to  ply  its 
tentacula  as  oars  on  either  side,  to  direct,  as  w^ell  as  acce- 
lerate its  motion.  No  sooner  does  the  breeze  freshen,  and 
the  sea  become  ruffled,  than  the  animal  hastens  to  take  down 
its  sail,  and  quickly  withdrawing  its  tentacula  within  its 
shell,  renders  itself  specifically  heavier  than  the  water,  and 
sinks  immediately  into  more  tranquil  regions  beneath  the 
surface.* 

The  common  Nautilus,  which  is  provided  with  a  similar 
sailing  apparatus,  is  an  inhabitant  of  a  polythalamous  shell, 
(Fig.  126,)  of  which  Fig.  127  represents  the  section.  The 
formation  of  this,  as  well  as  of  other  shells  of  this  descrip- 
tion, presents  very  curious  phenomena.  The  animal  at  cer- 
tain periods  of  its  growth,  finding  itself  cramped  in  the  nar- 

*  It  must  be  confessed,  however,  that  the  habits  of  the  Arg-onaut  are  still 
ver)^  imperfectly  known.  Considerable  doubts  are  entertained  whether  the 
shell  it  inhabits  is  formed  by  the  animal  itself,  or  whether  it  is  the  production 
of  some  other,  but  unknown  species  of  Mollusca,  and  is  merely  taken  pos- 
session of  by  the  Arg-onaut  as  a  convenient  habitation,  which  it  can  quit  and 
enter  again  at  pleasure. 


192  THE  MECHANICAL  FUNCTIONS. 

row  part  of  the  spire,  draws  up  that  portion  of  the  mantle 
which  occupied  it,  thus  leaving  a  vacant  space.  The  sur- 
face of  the  mantle  which  has  receded,  immediately  begins 
to  secrete  calcareous  matter,  which  is  deposited  in  the  form 
of  a  partition,  stretching  completely  across  the  area  of  the 
cavity.  As  the  animal  proceeds  to  increase  in  size,  and  to 
occupy  a  wider  portion  of  the  external  shell,  the  same  ne- 
cessity soon  recurs,  and  the  same  expedient  is  again  resort- 
ed to.  It  withdraws  its  mantle  from  the  narrower  into  the 
wider  part  of  the  sheH;  and  then  forms  a  second  partition, 
at  a  little  distance  from  the  first,  corresponding  to  the  space 
left  by  the  receding  of  the  mantle.  This  process  is  repeat- 
ed at  regular  intervals,  and  produces  the  multitude  of  cham- 
bers contained  in  polythalamous  shells,  of  which  the  living 
animal  occupies  only  the  largest,  or  that  which  continues 
open.*  The  partitions  are  in  general  perforated  either  in  the 
centre  or  at  one  side,  for  the  purpose  of  giving  passage  to  a 
ligament,  which  preserves  the  attachment  of  the  mantle  to 
the  apex  of  the  shell.  This  ligament  is  often  surrounded 
either  entirely  or  partially  by  shell,  which  forms  a  tube, 
denominated  the  syphon:  and  portions  of  which  are  seen  in 
the  section  Fig.  127. 

*  This  structure  is  extremely  prevalent  in  fossil  shells:  some  of  which  are 
spiral,  such  as  the  Cornu  Ammonis,  while  others  are  straight  cones,  such  as 
the  Bacculite  and  Orthoceratite.  In  most  of  tliese  the  partitions  are  very  nu- 
merous,  and  have  undulating-  surfaces. 


(      193      ) 


0 


CHAPTER  IV. 


ARTICULATA, 

§  1.  *drticulatad  Animals  in  general. 

From  the  Cephalopoda,  the  transition  is  easy  to  the  low- 
est order  of  vertebrated  animals.  But  previously  to  pur- 
suing the  analogies  which  connect  these  two  divisions  of  the 
animal  kingdom,  we  have  to  pass  in  review  a  ^^vj  exten- 
sive series  of  animal  forms,  constructed  upon  a  peculiar  sys- 
tem, and  occupying,  as  well  as  the  Mollusca,  a  place  inter- 
mediate between  Zoophytes  and  the  more  highly  organized 
classes. 

We  have  seen  that  even  in  those  Zoophytes  which  arc 
distinguished  from  the  rest  by  a  more  elaborate  conforma- 
tion of  organs,  the  powers  of  progressive  motion  are  always 
extremely  limited.     Nor  are  the  JNIollusca  in  general  more 
highly  favoured  with  respect  to  the  degree  in  which  they  en- 
joy this  faculty.     But  the  greater  number  of  the  animals 
composing  the  series  we  are  now  to  examine  are  provided 
with  a  complete  apparatus  for  motion,  and  endowed  with 
extensive  ca])acities  for   using  and  applying  it  in  various 
ways.     While  nature  has  preserved  in  the  construction  of 
their  vital  organs  the  simplicity  which  marks  the  primitive 
modes  of  organization,  and  has  adhered  to  a  definite  model 
in  the  formation  of  the  different  parts  of  the  system,  she  has 
nowhere  displayed  more  boundless  variety  in  the  combi- 
nations of  the  forms  whicli   she  has   impressed  upon  the 
mechanical  instruments,  both  of  prehension  and  of  progres- 
sion. 

All  the  tribes  of  Zoophytes,  and  by  far  the  greater  num- 
VoL.  I.  25 


194  THE  MECHANICAL  J'UNCTIONiJ. 

• 

ber  of  Mollusca,  are  limited  by  the  constitution  of  tbeir 
system,  to  an  aquatic  existence.  But  in  following  the  series 
of  Articulated  animals,  we  very  soon  emerge  from  the 
waters,  and  find  structures  adapted  to  progression  on  land. 
For  this  we  see  that  preparation  is  early  made  in  the  de- 
velopment of  the  nascent  structures.  A  farther  design,  also, 
soon  becomes  manifest;  and  instruments  are  given  for  ele- 
vating the  body  above  tlie  ground,  and  for  traversing  with 
rapidit}^  the  light  and  scarcely  resisting  atmosphere.  This 
prospective  design  may  be  traced  in  the  whole  system  of  in- 
sects; every  part  of  which  is  framed  with  reference  to  the 
properties  of  the  medium  through  which  these  movements 
are  to  be  performed. 

§  2.  Annelida, 

The  lowest  division  of  articulated  animals  comprehends 
those  which  have  a  vermiform  shape,  and  which  compose 
the  class  of  Annelida,  or  Annulose  animals;  of  which  the 
earth-worm  may  be  taken  as  the  type,  and  most  familiar  ex- 
ample. In  the  series  of  structures  which  constitute  this  di- 
vision of  the  animal  kingdom,  we  may  trace  remarkable  gra- 
dations of  development,  through  which  nature  appears  to 
pass  in  attaining  the  higher  and  more  perfect  conformations. 

It  may  be  remarked  that,  in  effecting  the  transition  from 
Zoophytes  to  the  new  model  of  construction  here  presented, 
nature  seems  to  have  wholly  abandoned  that  radiated  dispo- 
sition of  parts,  and  those  star-like,  forms,  so  characteristic 
of  the  beings  which  are  placed  on  the  confines  of  the  ani- 
mal kingdom,  and  which  still  retain  an  analogy  with  vege- 
table structures.  She  now  adopts  a  more  regular  law  of 
symmetry;  by  which  all  the  parts  are  referrible  to  one  lon- 
gitudinal axis,  and  also  to  a  vertical  plane  passing  through 
that  axis,  and  which  has  been  termed  the  mesial  plane.  As 
a  direct  consequence  of  this  law,  we  shall  find  that  in  the 
forms  which  are  hereafter  to  pass  under  our  review,  as  far 
as  the  external  organs  and  general  outline  of  the  body  are 
concerned,  all  that  exists  on  one  side  is  an  exact  counterpart, 
like  a  reflected  image,  of  what  is  found   on   the  other  side. 


ANNELIDA. 


195 


While  in  the  Star-fish,  and  Ecliinus,  nothing;  in  point  of  si- 
tuation was  delinite,  excepting  tlie  upper  and  the  lower  sur- 
face, and  there  was  no  side  which  could  be  exclusively  de- 
nominated either  the  right  or  the  left  side,  and  no  end  that 
could  be  properly  said  to  be  the  front  or  the  back,  in  Ar- 
ticulated as  well  as  in  Vertebrated  animals,  all  these  distinc- 
tions are  clearly  marked  and  easily  defined. 

In  all  the  Jlnnelida  the  firmest  parts  of  the  body,  or  those 
which  give  mechanical  support  to  the  rest,  are  external,  and 
may  be  regarded  either  as  appendages  to  the  integuments, 
or  as  modifications  of  the  integuments  themselves.  They 
consist  of  a  frame-work,  composed  of  a  series  of  horny  bands 
or  rings:  their  assemblage  having  more  orless  of  a  lengthened 
cylindric  shape,  and  constituting  a  kind  of  external  skeleton, 
which  encloses  all  the  other  organs.  This  is  exemplified  in 
the  earth-worm;  in  the  Pont-obdella,  (Fig.  128,)  which  is  a 
species  of  leech;  and  in  the  Nereis,  (Fig.  120.)     These  rings 


give  rise  to  the  division  of  the  body  into  as  many  difierent 
segments.  In  some  cases,  however,  we  find  all  these  rings 
compressed  into  the  form  of  a  flat  oval  disk.  This  is  the 
case  in  the  Erpohdella,  of  which  Fig.  130  is  an  enlarged 
representation. 

In  general,  the  first  of  the  segments  into  which  the  body 
is  divided,  contains  the  principal  organs  of  sense,  and  is  suf- 
ficiently distinct  from  those  which  follow  to  entitle  it  to  the 
appellation  of  the  head;  while  the  Icngtiicned  prolongation 
of  the  opposite  extremity,  when  such  a  form  is  present,  may 
be  denominated  the  tail. 


196  THE  MECHANICAL  FUNCTIONS. 

The  rings  which  encircle  the  body  are  connected  lateral- 
ly by  a  looser  and  more  flexible  portion  of  integument,  and 
also  by  layers  of  muscular  fibres,  curiously  collected  into 
bands.  The  muscular  flesh  of  insects,  and  other  animals  of 
this  class,  differs  much  from  that  of  the  larger  animals,  being 
soft  and  gelatinous  in  its  texture,  though  endowed  with  a 
high  degree  of  irritability,  and  contracting  with  great  force. 
The  fibres  composing  each  band  are  all  parallel  to  one  ano- 
ther, and  have  seldom  any  tendinous  attachments;  being  ge- 
nerally inserted  directly  on  the  parts  they  are  destined  to 
move.  Thus,  the  adjacent  margins  of  the  rings  of  worms, 
(as  shown  in  the  diagram.  Fig.  131,)  are  connected  together 
by  these  muscular  bands,  which  pass  transversely  from  the 
one  to  the  other,  immediately  under  the  skin,  and  parallel 
to  the  axis  of  the  body.  There  are  generally  four  distinct 
bands  provided;  two  running  along  the  back,  and  two  along 
the  lower  part  of  the  body. 

The  effects  which  result  from  the  action  of  these  muscles 
are  such  as  might  easily  be  anticipated.  The  lower  set  must, 
when  contracting,  bring  the  rings  nearer  to  one  another  at 
that  lower  part;  and  when  the  whole  series  occupying  that 
situation  are  exerted  in  concert,  they  will  raise  the  body  in 
the  form  of  an  arch.  An  opposite  curvature  will  be  pro- 
duced by  the  contraction  of  the  uj^per  bands;  whereby  the 
back  will  be  bent  downwards,  and  both  ends  of  the  body 
raised.  In  proportion  as  the  two  bands,  situated  on  each  side, 
act  in  concert,  while  the  others  are  relaxed,  the  body  will 
be  bent  laterally  towards  that  side.  When  all  the  four  mus- 
cular bands  contract  together  equally,  their  joint  effect  will 
be  to  bring  the  rings  near  to  each  other,  and  to  contract  the 
length  of  the  worm;  the  skin  being  at  the  same  time  wrink- 
led and  swelled  out  between  the  rings. 

Other  muscular  bands,  attached  to  the  rings,  pass  from  the 
one  to  the  other  in  more  oblique  directions.  By  means  of 
these  muscles  the  rings  may  be  made  to  recede  at  some 
points,  while  they  approach  at  others;  so  that  the  body  may 
be  either  twisted  laterally  on  its  axis,  or  wholly  elongated, 
according  as  the  actions  of  these  oblique  muscles  are  partial- 
ly or  generally  exerted. 


ANNELIDA.  197 

The  skin  on  the  surface  of  the  earth-worm  is  furnished 
at  the  parts  where  it  covers  the  rings,  witli  very  minute  bris- 
tles, called  Setx,  by  means  of  which  the  animal  is  enabled 
to  fix  those  parts  on  the  ground,  while  the  other  portions  of  its 
body  are  in  motion.    Both  in  the  anterior  and  posterior  seg- 
ments, these  hairs  are  directed  towards  the  centre  of  the  ani- 
mal; while  those  on  the  middle  segments  are  perpendicular.* 
We  almost  constantly  find,  in  animals  belonging  to  the  or- 
der of  Annelida,  some  provision  of  this  kind.     Often  it  con- 
sists of  tufts  of  hair  regularly  disposed  in  rows  on  each  side 
of  the  under  surface.     In  the  Nei^eis  (Fig.  129,)  a  genus  of 
sea-worms,  there  are  often  above  a  hundred  pair  of  little  tufts 
of  strong  bristles:  and  between  these  we  find  tentacula  to  pre- 
vent the  animal  from  running  against  any  thing  by  which  it 
might  be  injured.  They  also  raise  the  body  from  the  ground; 
for  which  purpose,  as  they  are  used  under  water,  very  little 
support  is  necessary.!   Sometimes  the  whole  body  is  covered 
with  hair;  at  other  times  these  appendages  are  in  the  form 
of  hooks,  which,  of  course,  give  greater  power  of  clinging  to 
the  objects  on  which  they  fasten.     In  some,  again,  they  as- 
sume more  the  nature  of  feet,  of  which  they  exercise  durinc; 
progression  all  the  functions;  being  furnished  with  several, 
sets  of  muscles  for  adjusting  and  strengthening  their  actions. 
The  mode  by  which  an  animal  of  this  description  advances 
along  the  ground  is  very  simple.       It  first  protrudes  the 
head  by  the  elongation  of  the  foremost  segments  of  the  body, 
while  the  others  cling  to  the  earth  by  means  of  tlie  rings, 
and  also  of  the  bristles  and  other  appendages  to  the  integu- 
ments.     The  head  is  then  applied  to  the  ground,  and  made 
the  fixed  point,  and  the  segments  next  to  it,  which  had  been 
elongated,  are  now  contracted  by  the  action  of  tiicir  longi- 
tudinal muscles;  in  doing  which,  equal  portions  of  tlie  suc- 

*  As  an  instance  of  the  extraordinary  multiplicity  of  species  existing  in 
every  department  of  living-  nature,  I  may  here  notice,  that  of  the  common 
earth-worm,  ajiparcntly  so  uniform  in  its  shape,  Savig-ny  has  lately,  by  a 
closer  examination,  been  able  to  disting-uish  no  less  than  twenty-two  diflercnt 
species  amoni^  those  found  in  the  neighbourhood  of  Paris  alone. 

f  Home;  Lectures,  &c.  Vol  i.  p.  115. 


198 


THE  MECHANICAL  FUNCTIONS. 


ceeding  segments  are  necessarily  elongated:  these  are  next 
contracted;  and  so  on,  in  succession,  till  the  whole  is  brought 
forwards  to  the  head:  after  which  the  same  series  of  actions 
is  repeated,  beginning  with  the  advance  of  the  head.  Worms 
often  reverse  this  motion,  and  are  thus  enabled  to  move  back- 
wards, or  with  the  tail  foremost.* 

Great  variety  exists  in  the  forms  of  the  animals  referrible 
to  the  type  of  Annelida.  The  Gordius,  or  hair-worm,  (Fig. 
]  32,)  is  that  which  exhibits  the  greatest  development  in 
length  compared  with  the  breadth  of  the  body.  It  has  the 
form  of  a  very  long  and  slender  thread:  the  annular  structure 
being  indicated  only  by  very  slight  transverse  folds  of  the 
integuments.  No  external  members,  nor  even  tentacula, 
have^been  given  to  this  simplest  of  vermiform  animals. 


135 


Many  of  the  animals  of  this  class  being  soft  and  defence- 
less, are  obliged  to  consult  their  safety  by  retreating  into 
holes  and  recesses,  or  by  burrowing  in  the  sand  or  mud. 
One  genus  only,  the  Serpula  (Fig.  133,)  forms  for  itself  an 
external  shell,  which  is  shaped  into  a  spiral  tube.  Others, 
as  the  Sabella  and  the  Terebella,  accomplish  the  same  ob- 
ject by  collecting  grains  of  sand,  or  fragments  of  decayed 
shells,  or  other  substances,  which  they  agglutinate  together 
by  means  of  a  viscid  exudation,  so  as  to  form  a  firm  defen- 
sive covering,  like  a  coat  of  mail.  Fig.  134  shows  this 
rude  architecture  in  the  Terehella  conchilega.  These  co- 
verings, however,  composed  as  they  are  of  extraneous  ma- 

*  See  Home?  Lectures  on  Coipparalive  Anatomy,  Vol.  i.  p.  114, 


ANNELIDA.  l.QO 

terlals,  and  not  being  orjranin  produnllons  of  Ihr  animals 
themselves,  are  structures  wholly  foreign  to  their  systems. 
These  inhabitants  of  tubes,  the  Tubicolae  of  Cuvier,  are  gene- 
rally furnished  with  tentacula,  issuing  from  the  head,  which, 
when  the  rest  of  the  body  has  retired  within  the  tube,  is  the 
only  part  exposed. 

The  expedient  resorted  to  for  progressive  motion  by  the 
Lumbricns  mariniis  oi  lumn^us  {Jirenicola  j)isc(tioriim  of 
Lamarck,)  is  very  remarkable.*  This  worm,  depicted  in 
Fig.  135,  swarms  on  all  sandy  shores,  and  is  dug  up  in  great 
numbers  as  bait  by  the  fishermen.  It  bores  its  way  through 
the  sand  by  means  of  the  peculiar  construction  of  the  rin^-s 
of  its  head,  which,  when  elongated,  has  the  shape  of  a  re- 
gular cone.  As  each  ring  is  so  much  smaller  than  the  one 
behind  it  as  to  admit  of  being  received  within  it,  the  whole 
head,  when  completely  retracted,  presents  a  flat  surface. 
When  this  disk  is  applied  to  the  sand,  the  animal,  by  gradu- 
ally projecting  the  cone,  and  successively  dilating  the  rings 
of  which  it  is  composed,  opens  for  itself  a  passage  through  the 
sand,  and  then  secures  the  sides  of  the  passage  from  falling  in 
by  applying  to  them  a  glutinous  cement,  which  exudes  from 
its  skin,  and  which  unites  the  particles  of  sand  into  a  kind  of 
wall,  or  coating.  This  covering  does  not  adhere  to  the  body, 
but  forms  a  detached  coherent  tube,  within  which  the  animal 
moves  with  perfect  freedom,  and  which  it  leaves  behind  it 
as  it  progressively  advances:  so  that  the  passage  is  kept  per- 
vious throughout  its  whole  length  by  means  of  this  lining, 
which  may  be  compared  to  the  brick  work  of  the  shaft  of 
a  mine,  or  tunnel. 

An  apparatus  of  a  more  complex  description  is  provided 
in  the  Terebellx  conchilegx,  belonging  to  a  tribe  of  marine 
worms,  which  from  the  peculiar  circumstances  of  their  situa- 
tion, inhabiting  parts  of  the  shore  nearly  midway  between 
high  and  low  water,  are  obliged  often  to  prolong  their  tubes 
to  a  great  length  through  the  sand;  for,  in  consequence  of 
the  frequent  shifting  of  tiic  sands  in  storms,  these  animals  are 

♦  See  the  account  given  by  Mr.  Osier,  Philosophical  Ti-ansaclions  fov 
1826,  p.  342. 


200  THE  MECHANICAL  FUNCTIONS. 

sometimes  buried  to  a  considerable  depth,  and  at  others  have 
several  inches  of  their  tubes  exposed.  In  the  one  case,  they 
must  work  their  way  speedily  to  the  surface;  in  the  other, 
they  must  dive  deeper  below  it.  The  manoeuvres  of  the 
terebella  are  best  observed  by  taking  it  out  of  its  tube  and 
placing  it  under  water  upon  sand.  It  is  then  seen  to  unfold 
all  the  coils  of  its  body,  to  extend  its  tentacula  in  every  di- 
rection, often  to  a  length  exceeding  an  inch  and  a  half,  and 
to  catch,  by  their  means,  small  fragments  of  shells,  and  the 
larger  particles  of  sand.  These  it  drags  towards  its  head, 
carrying  them  behind  the  scales  which  project  from  the  an- 
terior and  lower  part  of  the  head,  where  they  are  immediate- 
ly cemented  by  the  glutinous  matter  which  exudes  from  that 
part  of  the  surface.  Bending  the  head  alternately  from  side 
to  side,  while  it  continues  to  apply  the  materials  of  its  tube, 
the  terebella  has  very  soon  formed  a  complete  collar,  which 
it  sedulously  employs  itself  to  lengthen  at  every  part  of  the 
circumference  with  an  activity  and  perseverance  highly  in- 
teresting. For  the  purpose  of  fixing  the  different  fragments 
compactly,  it  presses  them  into  their  places  with  the  erected 
scales,  at  the  same  time  retracting  the  body.  Hence  the 
fragments,  being  raised  by  the  scales,  are  generally  fixed  by 
their  posterior  edges,  and  thus  overlaying  each  other,  often 
give  the  tube  an  imbricated  appearance. 

Having  formed  a  tube  of  half  an  inch,  or  an  inch  in  length, 
the  terebella  proceeds  to  burrow;  for  which  purpose  it  directs 
its  head  against  the  sand,  and  contracting  some  of  the  poste- 
rior rings,  effects  a  slight  extension  of  the  head,  which  thus 
slowly  makes  its  way  through  the  mass  before  it,  availing 
itself  of  the  materials  which  it  meets  with  in  its  course,  and 
so  continues  to  advance  till  the  whole  tube  is  completed. 
After  this  has  been  accomplished,  the  animal  turns  itself 
within  the  tube,  so  that  its  head  is  next  to  the  surface,  ready 
to  receive  the  water  which  brings  it  food,  and  is  instrumental 
in  its  respiration.  In  summer,  the  whole  task  is  completed 
in  four  or  five  hours;  but  in  cold  weather,  when  the  worm  is 
more  sluggish,  and  the  gluten  is  secreted  more  scantily,  its 
progress,  is  considerabl}^  slower. 


ANNELIDA.  201 

Tentacula  of  various  kinds  arc  also  met  witli  in  several  of 
the  more  active  and  vivacious  kinds  of  Annelida,  such  as  the 
Nereis  (Fig.  129,)  proceeding  from  the  margin  of  the  mouth 
and  other  parts  of  the  head.  This  animal  swims  with  great 
facility  by  rapid,  undulating  inflections  of  its  body;  and  by 
practising  a  similar  succession  of  movements  in  the  loose  sand 
at  the  bottom  of  the  water,  it  quickly  buries  itself,  and  even 
travels  to  considerable  distances  through  the  sand,  first  ex- 
tending the  anterior  rings,  and  then  bringing  up  the  poste- 
rior part  of  the  body;  its  progress  being  also  much  assisted 
by  the,  action  of  its  numerous  bristly  feet."* 

Facilities  for  progression  are  also  given  by  the  addition 
of  tubercles,  arranged  in  pairs  along  the  under  side  of 
the  body,  which  serve  the  purposes  of  feet,  and  are  often 
furnished  with  bristles  or  hooks.  In  the  Jimphilrite^  and 
many  other  genera,  tufts  of  hair  occupy  the  place  of  feet 
on  each  side,  and  being  moved  by  muscles  specially  provided 
for  that  purpose,  serve  as  levers  for  effecting  progressive 
motion. 

We  find  the  same  object  accomplished  by  very  different 
means  in  other  animals  of  this  class.  The  leech,  for  instance, 
having  the  rings  which  encircle  its  body  very  numerous  and 
close  to  each  other,  could  not  well  have  advanced  by  the  or- 
dinary modes  of  vermiform  progression.  As  a  substitute, 
accordingly,  it  has  been  furnished  with  an  apparatus  for  suc- 
tion at  the  two  extremities  of  the  body,  which  are  formed 
into  disks  for  that  purpose.  By  fixing  alternately  the  one 
and  the  other,  and  contracting  or  elongating  the  body  as  the 
occasion  requires,  the  leech  can  move  at  pleasure  either  for. 
wards  or  backwards.  Thus,  while  the  tail  is  fixed,  the  head 
may  be  advanced  by  lengthening  the  whole  body,  and  when 
the  head  is  fixed,  the  hinder  sucker  can  be  brought  forwards 
by  the  contraction  of  the  body,  and  applied  to  the  ground 
near  to  the  head,  and  preparation  may  thus  be  made  for 
taking  another  step. 

Most  of  the  parasitic  animals  which  inhabit  the  interior 

•  Osier,  Phil.  Trans.  1826,  p.  342. 
Vol.  I.  26 


202  .    THE  MECHANICAL  FUNCTIONS. 

cavities  of  the  body,  and  especially  the  alimentary  canal, 
correspond  in  external  form,  as  well  as  in  many  circum- 
stances of  internal  conformation,  to  the  Annelida.  They 
compose  an  order  denominated  the  Entozoa. 

§  3.  Arachnida. 

In  passing  from  the  Annelida  to  the  Arachnida^  an  order 
which  comprehends  all  the  species  of  spiders,  together  with 
animals  allied  to  them  in  conformation,  we  find  that  a  conside- 
rable advance  has  been  made  in  the  progress  of  development. 
The  frame-work  of  the  body  is  more  consolidated:  and  the 
instruments  provided  for  progressive  motion  are  shaped  into 
longer  and  more  perfect  levers,  are  united  by  a  more  refined 
system  of  articulation,  and  are  moved  by  more  distinct  and 
more  powerful  muscles;  so  that  the  body  is  elevated  from 
the  ground,  and  enjoys  a  greater  range  of  action,  and  a  wider 
sphere  of  perception. 

The  rings,  which  always  compose  the  frame-work  of  the 
Annelida,  are  here  consolidated  so  as  to  form  two  principal  di- 
visions of  the  body,  the  one  in  front,  termed  the  Cephalo-tho- 
rax,  which  contains  the  organs  of  sensation,  and  of  mastica- 
tion, and  also  the  principal  reservoir  of  circulating  fluids; 
the  other,  which  is  behind,  and  contains  the  organs  of  diges- 
tion, is  termed  the  abdomen.  In  the  spider  (Fig.  136,  where 

c  is  the  cephalo-thorax,  and  a 
the  abdomen)  these  two  por- 
tions of  the  body  are  separated 
by  a  deep  groove,  which  leaves 
only  a  slender  pedicle,  or  tube  of 
communication  between  them. 
There  are  usually  in  the  male 
four  pairs  of  legs,  constantly  articulated  with  the  cephalo-tho- 
rax; but  the  female  is  furnished  with  an  additional  pair  to  ena- 
ble her  to  carry  her  eggs.  For  the  purpose  of  obtaining  an  ex- 
tensive base  of  support,  the  feet  of  the  spider  are  spread  out  in 
diverging  rays,  so  as  to  include  a  very  wide  circle.  They  are 
divided  into  several  joints,  those  next  to  the  body  being  termed 


ARACHNIDA.  203 

the  haunches,  and  the  succeeding  ones  the  leg,  and  the  tarsus, 
and  each  foot  is  terminated  by  two,  or  sometimes  three  hooks. 
Besides  these,  there  are  other  members,  resembling  feet, 
which  are  placed  in  front  of  the  head,  and  have  affixed  to 
them  either  a  moveable  hook,  or  pincers,  which  are  employed 
as  organs  of  prehension,  and  of  offence.  Through  the  larger 
branches  of  these  a  canal  passes,  which  opens  near  the  point, 
and  conducts  a  poisonous  fluid  into  the  wounds  inflicted  by 
this  formidable  weapon. 

In  common  with  all  articulated  animals,  spiders,  in  the 
progress  of  their  growth,  cast  off*  their  outer  skin  several 
times,  and  at  regular  periods.  In  the  earlier  stages  of  their 
existence,  although  they  have  the  general  form  of  the  ma- 
ture insect,  yet  they  have  a  smaller  number  of  legs:  the  last 
pair  not  making  their  appearance  till  after  the  spider  has  at- 
tained a  certain  size.  We  may  here  trace  the  commence- 
ment of  that  system  of  metamorphosis,  w^hich,  as  we  shall  af- 
terwards find,  is  carried  to  so  great  a  length  in  winged  insects. 

Spiders  are  endowed  with  extensive  powers  of  progres- 
sive motion,  and  display  great  activity  and  energy  in  all 
their  movements.  The  long  and  elastic  limbs  on  which  the 
body  is  suspended,  being  firmly  braced  by  their  articulations, 
enable  the  muscles  to  act  with  great  mechanical  advantage 
in  accelerating  the  progression  of  the  body.  Hence,  these 
animals  are  enabled  to  run  with  great  swiftness,  and  to 
spring  from  a  considerable  distance  on  their  pre}';  powers 
which  were  necessary  to  those  tribes  that  live  altogether  by 
the  chase.  The  greater  number  of  species,  however,  as  is 
well  known,  are  provided  with  a  curious  apparatus  for 
spinning  threads,  and  for  constructing  webs  to  entangle  flies 
and  other  small  insects.  Every  species  of  spider  weaves 
its  web  in  a  manner  peculiar  to  itself:  and,  besides  the  prin- 
cipal web,  they  often  construct  in  the  neighbourhood  a 
smaller  one,  in  the  form  of  a  cell,  in  which  they  conceal 
themselves,  and  lie  in  ambush  for  their  prey.  Between 
this  cell  and  the  principal  web  they  extend  a  thread  of  com- 
munication, and  by  the  vibrations  into  which  this  thread  is 
thrown,  on  the  contact  of  any  solid  body,  the  spider  is  im- 


204  THE  MECHANICAL  FUNCTIONS. 

mediately  acquainted  with  the  event,  and  passes  quickly  to 
the  spot,  by  the  assistance  of  the  same  thread. 

Some  species  have  the  power  of  conveying  themselves  to 
considerable  distances  through  the  air  by  means  of  threads 
which  they  dart  out,  and  which  are  borne  onwards  by  the 
wind,  while  the  spider  is  clinging  to  the  end  of  the  thread 
^which  is  next  to  it.  In  this  manner  these  spiders  are  often 
carried  up  to  a  great  height  in  the  air:  and  it  has  been  sup- 
posed that  during  their  flight  they  often  seize  upon  gnats  and 
other  flies;  because  the  mutilated  remains  of  these  insects  are 
often  seen  adhering  to  the  threads:  this  point,  however,  is 
still  open  to  much  doubt. 

The  Natural  History  of  the  spider  is  in  many  points  of 
view^,  highly  interesting,  not  only  from  the  great  extent  to 
which  the  organic  development  is  carried,  and  the  energy 
with  which  all  the  functions  of  animal  life  are  performed; 
but  also  with  reference  to  the  wonderful  instincts  displayed 
in  the  construction  of  its  w^eb,  in  the  surprise  and  destruc- 
tion of  its  victims,  and  in  the  zealous  guardianship  of  its 
young.  It  would,  be  impossible  in  so  brief  an  outline  as  the 
one  I  am  now  tracing,  to  enlarge  upon  so  fertile  a  topic, 
without  being  led  too  far  from  the  object  I  have  at  present 
more  particularly  in  view;  namely,  the  development  of  or- 
ganization with  reference  to  the  organs  of  progressive  mo- 
tion. 


§  4.  Crustacea. 

The  plan  which  Nature  appears  to  have  commenced  in 
the  construction  of  the  Arachnida,  is  farther  pursued  in  that 
of  the  Crustacea.  The  portions  into  which  the  external 
frame-work  of  the  body  was  divided  in  the  former,  are  still 
farther  consolidated  in  the  latter:  they  are  composed  of 
denser  materials,  and  endowed  with  greater  rigidity;  thus 
not  only  offering  more  resistance  to  external  forces,  but  also 
giving  a  firm.er  purchase  to  the  muscles  which  are  the  moving 
powers.  The  lirnbs,  as  well  as  the  whole  body,  are  incased 
in  tubes  of  solid  carbonate  of  lime:  they  are  articulated  with 


CRUSTACEA. 


205 


great  care,  and  almost  always  compose  hinge  joints.  The 
muscles,  by  which  these  solid  levers  arc  moved,  are  lodged 
in  the  interior,  and  their  fd^res  either  pass  directly  from  one 
point  to  another,  across  the  joint;  or  else  they  are  attached 
to  cartilaginous  plates,  which,  for  the  purpose  of  receiving 
the  muscles,  are  made  to  project  intothe  interior  of  the  up- 
per portion  of  the  limb,  being  themselves  immoveably  con- 
nected with  the  lower  portion.  By  this  expedient,  not  only 
is  the  employment  of  a  tendon  dispensed  with,  but  a  larger 
surface  is  presented  for  the  attachment  of  the  muscles,  which 
by  acting  also  upon  a  longer  lever,  obtain  great  mechanical 
advantage.  It  would  be  superfluous  to  occupy  more  time  in 
explaining  the  minutiae  of  structure  in  these  joints,  because 
the  simple  inspection  of  the  limbs  of  a  crab  or  lobster  will 
give  clearer  ideas  of  this  mechanism  than  can  be  conveyed 
by  any  laboured  description.  We  must  content  ourselves 
with  a  brief  sketch  of  the  principal  constituent  parts  of  these 
external  members  of  the  Crustacea. 

The  number  of  pairs  of  legs  is  either  three  or  four:  each 
leg  is  divided  into  five  pieces.     The  piece  ii,  (Fig.  137,) 


next  the  trunk,  is  termed  the  haunch,  to  which  is  united 
the  trochanter,  t;  after  which  comes  in  succession  the  fe- 
mur or  thigh,  f;  two  portions  of  the  leg,  l;  and  the  tarsus 
p.  The  haunch  is  usually  short,  being  interposed  merely  as 
a  base  for  increasing  the  extent  of  motion  of  the  pieces 
which  follow;  and  sometimes  it  is  itself  composed  of  more 


206  THB  MECHANICAL  FUNCTIONS. 

than  one  piece.  The  leg  is  usually  divided  into  two  pieces, 
by  a  joint.  The  tarsus  is  terminated  by  a  single  or  double 
hook,  and  sometimes  by  a  pincer,  or  claw. 

New  organs,  not  met  with  among  the  Arachnida,  are  here 
for  the  first  time  developed,  namely,  the  Antennse^  of  which 
there  is  one  on  each  side  of  the  head.  They  are  denomi- 
nated, in  popular  language,  the  feelers;  although  it  is  more 
than  probable  that  they  perform  some  function  of  higher 
^importance  than  that  of  conveying  perceptions  of  mere 
touch.  The  antennae  consist  of  slender  filaments,  composed 
of  a  great  number  of  pieces  articulated  together:  and  they 
are  infinitel}^  diversified  in  their  form  in  the  different  genera 
and  species,  both  of  Crustacea  and  of  Insects. 

The  jaws,  and  other  parts  connected  with  the  mouth,  pre- 
sent a  great  complication  of  structure;  and  many  of  these 
parts  are  employed  in  various  uses  besides  those  of  mastica- 
tion; such  as  the  seizing  of  objects,  and  turning  them  in  va- 
rious ways  for  examination;  and,  according  to  their  suita- 
bleness as  articles  of  food,  conveying  them  into  the  mouth. 
These  organs  are  called  the  Palpi,  and  sometimes  ih^  false 
feet.     They  always  exist  in  pairs,  and  take  their  rise  from 
the  lower  lip,  or  some  adjacent  part  of  the  head.     The  por- 
tions of  which  each  is  composed  are  articulated  together  and 
moved  by  muscles  in  the  same  manner  as  the  ordinary  or 
proper  feet.     It  is  worthy  of  notice,  however,  that  some- 
times the  foremost  pairs  of  palpi  are  shaped  more  like  jaws, 
and  actually  perform  the  office  proper  to  jaws,  of  compress- 
ing and  dividing  the  food  previously  to  its  introduction  into 
the  mouth.     These  auxiliary  jaws  are  then  called  mandibles. 
In  other  instances,  we  see  them  assuming  every  variety  of  in- 
termediate form  between  that  of  mandibles  and  of  false  feet, 
so  that  it  is  often  difficult,  amidst  these  gradual  transitions 
of  structure,  to  decide  to  which  of  these  two  kinds  of  organs 
a  specimen  we  meet  with  properly  belongs.     It  is  apparent- 
ly with  a  view  to  evade  this  difficulty  that  a  term  has  been 
invented  which  shall  include  them  all,  namely,  that  oi feet- 
jaws.     These   transitions  are    illustrated    by   the  annexed 
figures  of  several  of  these  members  in  the  Mysis  Fabricii; 


CRUSTACEA.  097 

Fig.  138,  being  that  of  a  mandible,  with  its  feeler,  or  palpus; 
Figures  139,  140,  and  141,  representing  the  fust,  second, 
and  third  pairs  of  feet-jaws;  and  Fig.  142,  the  first  pair  of 
true  feet.  It  would  thus  seem  as  if  the  same  constituent  ele- 
ment of  the  fabric  is  converted  by  nature  into  the  one  or  other 
of  these  organs,  according  as  best  suits  the  exigencies  of  each 
particular  case.'^ 

In  the  lobster,  the  crab,  and  many  other  analogous  Crus- 
tacea, the  foremost  pair  of  true  feet  are  also  modified  to  suit 
a  particular  purpose;  the  pincers  which  terminate  them  being 
expanded  into  a  claw,  and  constituting  a  powerful  oro-an  of 
prehension,  and  a  formidable  weapon  of  offence.  It  resem- 
bles a  finger  and  thumb  in  its  power  of  grasping  and  strongly 
compressing  any  object  on  which  it  seizes;  and,  to  enable  it 
to  do  this  with  more  effect,  the  inner  edges  of  both  parts  of 
the  claw  are  notched  or  serrated. 

The  large  portion  of  shell  which  is  consolidated  into  one 
piece,  and  covers  the  upper  part  of  the  body,  is  termed  the 
shield,  or  carapace.     The  tail  of  the  crab  is  very  short,  and 
is  united  with  the  body,  appearing  as  if  it  had  been  folded 
under  it.     The  feet-jaws  are  particularly  large,  but  short: 
the  articulations  of  the  feet  are  such  as  to  allow  of  scarcclv 
any  motion  but  in  a  transverse  plane.     This  is  the  cause  of 
the  greater   facility  the  crab   finds  in  walking  side-ways, 
which  it  can  do  with  great  quickness  when  urged  by  a  sense 
of  danger.     The  lobster,  on  the  contrary,  is  better  formed 
for  swimming  than  for  walking.     The  hinder  part  of  its 
body  is  divided  into  segments,  which  play  upon  each  other 
by  a  remarkable  kind  of  m.echanism,  the  margins  of  each 
portion  overlapping  the  succeeding  segment,  and  partly  en- 
closing it.     The  tail  is  the  principal  agent  used  in  swim- 
ming, and  the  whole  force  of  the  muscles  is  bestowed  upon 
its  movements.     As  it  strikes  the  water  from  behind  for- 

•  The  labours  ofSavigiiy,  Audouin  and  Latreille  appear  to  have  established 
a  complete  analogy  in  the  respective  component  parts,  not  only  of  the  feet, 
feet-jaws,  jaws  and  mandibles,  but  also  of  the  palpi  and  other  appendices  at- 
tached  to  the  head,  in  all  the  articulated  animals,  whether  belonging  to  the 
closes  of  arachnlda,  crustacca,  myriapoda,  or  winged  insects. 


20S  THE  MECHANICAL  FUNCTIONS. 

wards,  the  lobster  can  only  swim  backwards;  and  it  is  as- 
sisted in  this  action  by  five  pair  of  false  feet,  which  are  at- 
tached to  the  under  side  of  the  body,  behind  the  true  feet, 
and  w^hich  terminate  in  a  fin-shaped  expansion,  giving  them 
the  effect  of  oars.  The  extremity  of  the  tail  is  still  more 
expressly  formed  for  giving  effect  to  the  stroke,  being  ter- 
minated by  a  number  of  fiat  scales,  which,  when  expanded, 
present  a  broad  surface  to  the  water. 

The  calcareous  coverings  of  these  Crustacea  are  analogous 
to  shell  both  in  structure  and  composition.  They  contain, 
however,  some  phosphate  of  lime,  in  addition  to  the  carbo- 
nate. The  calcareous  particles  are  deposited  on  a  membrane 
of  considerable  firmness;  and  they  together  compose  a  dense, 
but  thin  and  fragile  structure,  which,  in  order  to  distinguish 
it  from  the  shells  of  the  mollusca,  has  been  denominated  a 
crust.  A  solid  structure  of  this  kind,  as  we  have  already 
seen,  does  not  admit  of  increase  by  the  extension  of  its  own 
parts:  so  that,  in  order  to  allow  of  the  growth  of  the  parts 
which  it  encloses,  it  is  necessary  that  it  be  cast  ofi',  and  ex- 
changed for  a  new  shell  of  larger  dimensions. 

The  process  by  w^hich  this  periodical  casting  and  renewal 
of  the  shell  are  effected,  has  been  very  satisfactorily  investi- 
gated by  Reaumur.  The  tendency  in  the  body  and  in  the 
limbs  to  expand  during  growth  is  restrained  by  the  limited 
dimensions  of  the  shell,  which  resists  the  efforts  to  enlarge 
its  diameter.  But  this  force  of  expansion  goes  on  increasing, 
till  at  length  it  is  productive  of  much  uneasiness  to  the  ani- 
mal, which  is,  in  consequence,  prompted  to  make  a  violent 
effort  to  relieve  itself;  by  this  means  it  generally  succeeds 
in  bursting  the  shell;  and  then,  by  dint  of  repeated  struggles, 
extricates  its  body  and  its  limbs.  The  lobster  first  with- 
draws its  claws,  and  then  its  feet,  as  if  it  were  pulling  them 
out  of  a  pair  of  boots:  the  head  next  throws  off  its  case,  to- 
gether with  its  antennas;  and  the  two  eyes  are  disengaged 
from  their  horny  pedicles.  In  this  operation,  not  only  the 
complex  apparatus  of  the  jaws,  but  even  the  horny  cuticle 
and  teeth  of  the  stomach,  are  all  cast  off  along  with  the  shell: 
and,  last  of  all,  the  tail  is  extricated.     But  the  whole  process 


CRUSTACEA.  209 

is  not  accomplished  without  long  continued  cfTorts.     Some- 
times the  legs  are  lacerated  or  torn  off,  in  the  attempt  to 
withdraw  them  from  the  shell;  and  in  the  younger  Crustacea 
the  operation  is  not  unfrequently  fatal.     Even  when  success- 
fully accom])lished,  it  leaves  the  animal  in  a  most  languid 
state:  the  limbs,  being  soft  and  pliant,  are  scarcely  able  to 
drag  the  body  along.     They  are  not,  however,  left  altoge- 
ther without  defence.     For  some  time  before  the  old  shell 
was  cast  off,  preparations  had  been  making  for  forming  a 
new  one.     The  membrane  which  lined  the  shell  had  been 
acquiring  greater  density,  and  had  already  collected  a  quan- 
tity of  liquid  materials  proper  for  the  consolidation  of  the 
new  shell.     These  materials  are  mixed  with  a  large  propor- 
tion of  colouring  matter,  of  a  bright  scarlet  hue,  giving  it 
the  appearance  of  red  blood,  thougli  it  differs  totally  from 
blood  in  all  its  other  properties.     As  soon  as  the  shell  is 
cast  off,  this  membrane,  by  the  pressure  from  within,  is  sud- 
denly expanded,  and  by  the  rapid  growth  of  the  soft  parts, 
soon  acquires  a  much  larger  size  than  the  former  shell. 
Then  the  process  of  hardening  the  calcareous  ingredient 
commences,  and   is  rapidly  completed;  while  an  abundant 
supply  of  fresh  matter  is  added  to  increase  the  strength  of 
the  solid  walls  which  are  thus  constructing  for  the  support 
of  the  animal.     Reaumur  estimates  that  the  lobster  gains, 
during  each  change  of  its  covering,  an  increase  of  one-fifth  of 
its  former  dimensions.     When  the  animal  has  attained  its 
full  size,  no  operation  of  this  kind  is  required,  and  the  same 
shell  is  permanently  retained. 

A  provision  appears  to  be  made,  in  the  interior  of  the  ani- 
mal, for  the  supply  of  the  large  quantity  of  calcareous  mat- 
ter required  for  the  construction  of  the  shell  at  the  proper 
time.  A  magazine  of  carbonate  of  lime  is  collected,  pre- 
vious to  each  change  of  shell,  in  the  form  of  two  rounded 
masses,  one  on  each  side  of  the  stomach.  In  the  crab 
these  balls  have  received  the  absurd  name  of  crab's  eyes;  and 
during  the  formation  of  the  shell  they  disap])car. 

It  is  well  known  that  when  an  animal  of  this  class  has 
been  deprived  of  one  of  the  claws,  that  part  is  in  a  short 

Vol.  I.  27 


210  THE  MECHANICAL  FUNCTIONS. 

time  replaced  by  a  new  claw,  which  grows  from  the  stump 
of  the  one  which  had  been  lost.     It  appears  from  the  inves- 
tigations of  Reaumur,  that  this  new  growth  takes  place  more 
readily  at  particular  parts  of  the  limb,  and  especially  at  the 
joints;  and  the  animal  seems  to  be  aware  of  the  greater  fa- 
cility with  which  a  renewal  of  the  claw  can  be  effected  at 
these  parts;  for  if  it  chance  to  receive  an  injury  at  the  ex- 
tremity of  the  limb,  it  often,  by  a  spontaneous  effort,  breaks 
off  the  whole  limb  at  its  junction  with  the  trunk,  which  is 
the  point  where  the  growth  more  speedily  commences.    The 
wound  soon  becomes  covered  with  a  delicate  white  mem- 
brane, which  presents  at  first  a  convex  surface:  this  gradu- 
allv  rises  to  a  point,  and  is  found  on  examination  to  conceal 
the  rudiment  of  a  new  claw.     At  first  this  new  claw  en- 
larges but  slowly,  as  if  collecting  strength  for  the  more  vigo- 
rous effort  of  expansion  which  afterwards  takes  place.     As  it 
o-rows,  the  membrane  is  pushed  forwards,  becoming  thin- 
ner in  proportion  as  it  is  stretched;  till  at  length  it  gives 
wMy,  and  the  soft  claw  is  exposed  to  view.     The  claw  now 
enlarges  rapidly,  and  in  a  few  days  more  acquires  a  shell  as 
hard  as  that  which  had  preceded  it.     Usually,  however,  it 
does  not  attain  the  same  size;  a  circumstance  which  accounts 
for  our  frequently  meeting  with  lobsters  and  crabs  wdiich 
have  one  claw  much  smaller  than  the  other.     In  the  course 
of  the  subsequent  castings,  this  disparity  gradually  disap- 
pears.    The  same  power  of  restoration  is  found  to  reside  in 
the  legs,  the  antennse,  and  the  jaws. 

We  must  naturally  be  curious  to  learn,  if  possible,  from 
what  source  these  astonishing  powers  of  regeneration  are  de- 
rived. Reaumur  hazarded  the  conjecture,  that  there  might  be 
orio^inally  implanted  in  each  articulation  a  certain  number  of 
embryo  limbs,  ready  to  be  developed  as  occasion  might  re- 
quire; somewhat  in  the  way  in  which  the  rudiments  of  the 
secondary  teeth  remain  concealed  in  the  jaw,  in  preparation 
for  replacing  the  first  set  when  these  have  been  removed. 
But  this  hypothesis  is  overturned  by  the  fact  that  if  the  ani- 
mal loses  only  part  of  the  limb,  it  is  the  deficient  portion 
alone,  and  not  the  whole  limb  that  is  regenerated.     The 


CRUSTACEA.  211 

sprouting  of.  the  new  claw  bears  a  strong  analogy  to  the 
budding  of  a  plant;  both  having  their  origin  from  an  imper- 
ceptible atom,  or  germ,  which  is  either  formed  on  the  oc- 
casion, or  had  pre-existed  in  the  organization.  We  are, 
however,  totally  destitute  of  the  means  of  deciding  which 
of  these  alternatives  is  nearest  to  the  truth.  It  is  but  too 
probable  that  the  agents  which  can  effect  such  wonderful 
operations  will  ever  baffle  our  most  scrutinizing  inquiries, 
and  that  they  are  of  too  refined  an  order  to  come  within 
the  reach  of  the  most  subtle  conjectures  that  human  imagi- 
nation can  devise. 


(     212      ) 


CHAPTER  V. 

INSECTS. 

§  1.  Apt  era. 

Apterous,  or  wingless  insects  form  the  next  term  in  the 
series  of  articulated  animals.  Closely  allied  in  their  organi- 
zation to  many  of  the  preceding  families,  they  differ  from 
them  in  being  essentially  formed  for  a  terrestrial  instead  of 
an  aquatic  life.  Most  of  the  lower  tribes  of  this  order  are 
parasitic,  that  is  derive  their  nourishment  from  the  juices 
of  other  animals,  the  skin  of  which  they  infest  and  penetrate, 
and  into  which  they  insert  tubes  for  suction.  The  various 
tribes  oi  JJcari,  or  mites,  of  Pediculi,  or  lice,  oi  Ricini,  or 
ticks,  of  Pulices^  or  fleas:  together  with  the  Podura,  or 
spring-tail;  the  Lepisma,  and  the  family  of  Myriopoda,  or 
millepedes,  are  comprehended  in  this  order.  I  shall  be 
obliged  to  pass  over  these  tribes  very  cursorily,  noticing 
only  a  few  of  the  more  remarkable  circumstances  attending 
their  mechanical  conformation. 

The  Pulex  is  the  only  apterous  insect  that  undergoes  com- 
plete metaphorphoses  in  the  course  of  its  development.  In 
the  first  stage  of  its  existence,  it  has  the  form  of  a  long 
worm,  without  feet,  frequently  rolling  itself  into  a  spiral 
coil.  It  consists  of  thirteen  segments,  having  tufts  of  hair 
growing  upon  each.  In  its  mature  state,  it  has  six  articu- 
lated legs,  the  hindmost  of  which  are  of  great  size,  for  the 
purpose  of  enabling  the  insect  to  take  those  prodigious  leaps 
which  astonish  us  in  beings  of  so  diminutive  a  size,  and  af- 
ford a  striking  proof  of  the  exquisite  mechanism  j^ervading 
even  the  lowest  orders  of  the  animal  creation. 

The  Podiira  leaps  into  the  air  by  a  mechanical  contri- 
vance of  another  kind;  employing  for  this  purpose  the  tail, 
which  is  very  long,  and  forked  at  the  end.     In  its  ordinary 


APTERA.  213 

state  this  organ  is  kept  folded  under  the  abdomen,  where  it 
is  concealed  in  a  groove.  The  pieces  of  whicli  it  is  com- 
posed are  articulated  together  in  such  a  manner  as  to  admit 
of  their  being  rapidly  unbent  by  the  action  of  its  muscles, 
the  whole  mechanism  conspiring  to  produce  the  effect  of  a 
powerful  spring,  by  which  the  body  is  propelled  forwards 
to  a  considerable  distance.  In  some  species,  this  flexible 
tail  has  a  flattened  form,  for  the  purpose  of  enabling  the  in- 
sect to  leap  from  the  surface  of  the  water,  an  action  which  it 
performs  with  apparently  as  much  ease  as  if  it  sprung  from 
a  solid  resisting  plane. 

The  Lapisma  leaps  by  means  of  moveable  appendages, 
placed  in  a  double  row  along  the  under  side  of  the  body,  and 
acting  like  springs.  There  are  eight  pairs  of  these  members, 
corresponding  in  situation  and  structure  to  the  false  feet  of 
the  Crustacea,  and,  like  them,  terminating  in  jointed  fda- 
ments. 

The  Juhis  and  the  Scolopendra,  which  compose  the  fa- 
mily of  the  Myriapoda,  so  called  from  the  immense  num- 
ber of  their  feet,  undergo,  to  a  certain  extent,  a  kind  of  me- 
taphorphosis  in  the  progress  of  their  development.  When 
first  hatched  they  have  often  no  feet  whatever,  and  resem- 
ble the  simpler  kinds  of  worms.  Legs  at  length  make  their 
appearance;  but  they  arise  in  succession,  and  it  is  not  until 
the  later  periods  of  their  growth  that  these  animals  acquire 
their  full  complement  of  segments,  with  their  accompanying 
legs.  The  Jidus  ierrestris,  for  example,  (Fig.  143)  has,  at 
^  its  entrance  into  the  world,  only 

jiii0^(4^^^^^j^^^^^^  eight  segments  and  six  feet;  but 
^^^^^Hll!lliP\  acquires  in  the  course  of  its  deve- 
lopment, fifty  segments  and  about  two  hundred  feet.  The 
anterior  legs  are  directed  obliquely  forwards,  and  the  rest 
more  or  less  backwards.  The  mandibles  have  the  form  of 
small  feet;  as  we  have  seen  is  frequently  the  case  in  crusta- 
ceous  animals. 


214 


THE  MECHANICAL  FUNCTIONS. 


§  2.  Insect  a  alata. 

Our  attention  Is  now  to  be  directed  to  the  more  highly 
developed  Insects,  which  have  been  formed  with  a  view  to 
progression  through  the  air.  On  these,  which  compose  the 
most  extensive  class  of  the  whole  animal  kingdom.  Nature 
has  lavished  her  choicest  gifts  of  animal  powers,  as  far  as 
they  are  compatible  with  the  diminutive  scale  to  which  she 
has  restricted  herself  in  their  formation.  The  model  she 
has  chosen  for  their  construction  is  that  which  combines  the 
greatest  security  against  injurious  impressions  from  without, 
with  the  most  extensive  powers  of  locomotion;  and  which 
also  admits  of  the  fullest  exercise  of  all  those  faculties  of 
active  enjoyment  which  are  characteristic  of  animal  life.  She 
has  provided  for  the  first  of  these  objects  by  enclosing  the 
softer  organs  in  dense  and  horny  coverings,  which  perform 
the  office  of  an  external  skeleton,  sustaining  and  protecting 
the  viscera,  and  furnishing  extensive  surfaces  of  attachment 
to  the  muscles,  from  the  action  of  which  all  the  varied  move- 
ments of  the  system  are  derived. 

The  muscular  system  of  perfect  insects  is  exceedingly 
complex.  Lyonet  has  described  and  delineated  an  immense 
number  of  muscular  bands  in  the  caterpillar  of  the  Cossus, 
and  the  plates  he  has  given  have  been  copied  in  a  variety 
of  books  in  illustration  of  this  part  of  the  structure  of  in- 
sects.    The   recent  work  of  Straus  Durckheim  affords  an 


equally  striking  example  of  admirable  arrangement  in  the 
muscles  of  the  Melolontha  vulgaris,  or  cockchaffer,  the  ana- 


WINGED    INSECTS.  215 

tomy  of  which  has  l)ccn  minutely  invcstigntcd  l)y  that  dis- 
tinguished entomologist.  These  muscles  are  represented  in 
Fig.  144,  which  has  heen  carefully  reduced  from  his  beauti- 
fully executed  plates.  The  largest  mass  of  muscular  fibres 
is  that  marked  a,  which  depress  the  wings,  and  are  of  enor- 
mous size  and  strength. 

On  examining  the  different  structures  which  compose  the 
solid  frame-work  of  insects,  we  find  them  conforming  in  every 
instance  to  the  general  type  of  annulose  animals,  inasmuch 
as  they  consist  of  thickened  portions  of  integument,  encir- 
cling the  body;  but  variously  united  and  consolidated,  for 
the  manifest  purpose  of  obtaining  greater  mechanical  strength 
and  elasticity  than  if  they  had  remained  detached  pieces, 
joined  only  by  membranous  connexions.  A  long  flexible 
body,  such  as  that  possessed  by  the  Myriapoda,  could  not 
easily  have  been  transported  through  the  air;  for  every  bend 
would  have  created  a  resistance,  and  have  impeded  its  ad- 
vance during  flight.  Hence  the  body  of  the  insect,  which 
is  to  be  ultimately  adapted  to  this  mode  of  progression,  has 
been  shortened  by  a  reduction  in  the  number  of  its  segments, 
and  rendered  more  simple  and  compact.  The  segments  des- 
tined to  support  tiie  wings  have  been  expanded  for  the  pur- 
pose of  lodging  the  powerful  muscles  that  are  to  move  them; 
and  rendered  dense  and  unyielding  in  order  to  sup])ort  their 
action. 

Nature  has  farther  provided  insects  with  instruments 
adapted  to  different  kinds  of  external  actions.  They  consist 
of  articulated  levers,  variouslv  combined  together,  and  form- 
ing  legs,  claws,  pincers,  oars,  palpi,  and,  lastly,  wings,  cal- 
culated for  executing  every  variety  of  prehension,  of  pro- 
gression, or  whatever  other  action  their  wants  and  necessities 
require. 

§  3.  Development  of  Insects. 

It  would  appear  as  if  the  final  accomplishment  of  objects 
so  numerous,  so  widely  difierent,  and  so  liable  to  mutual  in- 
terference, could  be  attained  only  by  the  animal  being  sub- 


216  THE  MECHANICAL  FUNCTIONS. 

jected  to  a  long  series  of  modifications,  and  passing  through 
many  intermediate  stages  of  development.  The  power  of 
flight  is  never  conferred  upon  the  insect  in  the  earlier  peri- 
ods of  its  existence:  for  before  its  structure  can  obtain  the 
lightness  which  fits  it  for  rising  in  the  air,  and  before  it  can 
acquire  instruments  capable  of  acting  upon  so  light  an  ele- 
ment, it  has  to  go  through  several  preparatory  changes,  some 
of  which  are  so  considerable  as  to  justify  the  term  oi  meta- 
Tnoiyhoses,  which  has  been  generally  given  to  them."^  But 
transient  is  the  state  of  perfection  in  every  thing  that  re- 
lates to  animal  existence.  When  the  insect  has  by  a  slow 
development  reached  this  ultimate  elaboration  of  its  organs, 
its  life  is  hastening  to  a  close;  and  the  period  of  its  perfect 
state  is  generally  the  shortest  of  its  whole  existence. 

The  history  of  the  successive  stages  of  development  of  in- 
sects opens  a  highly  interesting  field  of  philosophical  inqui- 
ry.    For  a  certain  period  of  the  early  life  of  these  animals, 
the  growth  of  all  the  parts  appears  to  proceed  equably  and 
uniformly:  but  at  subsequent  epochs,  some  parts  acquire  a 
great  and  sudden  increase  of  size,  and  others  that  were  in  a 
rudimental  condition  become  highly  developed,  and  consti- 
tute what  appear  to  be  new  forms  of  organs,  although  their 
elements  were  in  existence  from  a  much  earlier  period.    The 
modifications  which  the  harder  and  more  solid  structures  of 
insects  exhibit  in  the  progress  of  these  changes,  are  particu- 
larly remarkable,  as  illustrating  the  principles  on  which  the 
development  is  conducted.     The  researches  of  modern  en- 
tomologists have  led  to  the  conclusion  that  the  frame-work, 
or  skeleton  of  insects,  is  always  formed  by  the  union  of  a 
certain  determinate  number  of  parts,  or  elements,  originally 
distinct  from  one  another,  but  v/hich  are  variously  joined 
and  soldered  together  in  the  progress  of  growth:  frequently 
exhibiting  a  great  disproportion  in  the  comparative  expan- 
sion of  different  parts.     The  enlargement  of  any  one  part, 
however,  exercises  a  certain  influence  on  all  the  neighbour- 

*  Transformations  quite  as  remarkable  occur  in  several  tribes  of  animals 
belonging  to  other  classes:  such  as  those  of  the  Frog  among  reptiles,  and  of 
the  Lemsea  among  parasitic  worms. 


DEVELOPMENT  OP  INSECTS. 


o 


17 


ing  parts,  and  thus  are  the  foundations  laid  of  all  the  endless 
diversities  which  characterize  the  several  species  belonging 
to  each  tribe  and  family. 

In  the  progress  of  development,  we  may  recognise  two 
principles,  which,  though  apparently  opposite  to  each  other, 
concur  and  harmonize  in  their  operation:  these  are  expaii- 
sion  and  concentration.  Thus,  while  those  segments  of  body 
which  follow  the  head  are  greatly  enlarged,  in  order  to  sup- 
port the  more  recently  developed  organs  of  progressive  mo- 
tion, they  are  also  more  consolidated,  and  rendered  stronger 
by  the  union  of  several  pieces  which  were  before  separate. 
The  hinder  segments,  having  no  such  appendages  to  support, 
are  less  dilated,  and  the  whole  body  is  much  shortened  by 
the  approximation  of  the  segments,  which,  in  this  way,  com- 
pose the  abdomen,  or  hinder  division  of  the  insect. 

The  progress  of  the  metamorphoses  of  insects  is  most 
strikingly  displayed  in  the  history  of  the  Lepidopterous,  or 
butterfly  and  moth  tribe.*  The  egg,  which  is  deposited  by 
the  butterfly,  gives  birth  to  a  caterpillar;  an  animal,  which, 


in  outward  shape,  bears  not  the  slightest  resemblance  to  its 
parent,  or  to  the  form  it  is  itself  afterwards  to  assume.  It 
has,  in  fact,  both  the  external  appearance,  and  the  mechani- 
cal structure,  of  a  worm.     The  same   elongated  cylindric 

*  The  four  periods  of  the  existence  of  the  Bomhyx  mor'i,  or  the  moth  of 
the  silk-worm,  are  shown  in  the  annexed  cng-raviiigs:  Fig-.  145  are  tlie  ci,-gs; 
Fig.  146,  the  Larva,  or  caterpillar;  Fig-.  147,  the  Pupa,  or  chr^sidis;  and 
Fig.  148,  the  I?nagOy  or  perfect  insect. 

Vol.  I.  28 


218  THE  MECHANICAL  FUNCTIONS. 

shape,  the  same  annular  structure  of  the  denser  parts  of  Its 
integument,  the  same  arrangements  of  longitudinal  and  ob- 
lique muscles  connecting  these  rings,  the  same  apparatus  of 
short  feet,  with  claws,  or  bristles,  or  tufts  of  hairs,  for  faci- 
litating progression;  in  short,  all  the  circumstances  most 
characteristic  of  the  vermiformx  type  are  equally  exemplified 
in  the  different  tribes  of  caterpillars,  as  in  the  proper  An- 
nelida. 

But  these  vermiform  insects  have  this  peculiarity,  that 
they  contain  in  their  interior  the  rudiments  of  all  the  or- 
gans of  the  perfect  insect.  These  organs,  however,  are  con- 
cealed from  view  by  a  great  number  of  membranous  cover- 
ings, which  successively  invest  one  another,  like  the  coats  of 
an  onion,  and  are  thrown  off,  one  after  another,  as  the  inter- 
nal parts  are  gradually  developed.  These  external  invest- 
ments, which  hide  the  real  form  of  the  future  animal,  have 
been  compared  to  a  mask;  so  that  the  insect,  while  wearing 
this  disguise,  has  been  termed  larva,  which  is  the  Latin 
name  for  a  mask. 

This  operose  mode  of  development  is  rendered  necessary 
in  consequence  of  the  greater  compactness  of  the  integu- 
ments of  insects,  as  compared  with  those  of  the  annelida.   In 
proportion  as  they  acquire  density  they  are  less  capable  of 
being  farther  stretched,  and  at  length  arrive  at  the  limit  of 
their  possible  growth.     Then  it  is  that  they  obstruct  the  di- 
latation of  the  internal  organs,  and  must  be  thrown  off  to 
make  way  for  the  farther  growth  of  the  insect.     In  the 
mean  time  a    new    skin   has    been    preparing  underneath, 
moulded  on  a  larger  model,  and  admitting  of  greater  exten- 
sion than  the   one  which  preceded  it.     This   new  skin,  at 
first,  readily  yields  to  the  distending  force  from  within,  and 
a  new  impulse  is  given  to  the  powers  of  development:  un- 
til, becoming  itself  too  rigid  to  be  farther  stretched,  it  must, 
in  its  turn,  be  cast  off  in  order  to  give  place  to  another  skin. 
Such  is  the  process  which  is  repeated  periodically,  for  a 
great  number  of  times,  before  the  larva  has  attained  its  full 
size. 

These  successive  peelings  of  the  skin  are  but  so  many 


DEVELOPMENT  OF  INSECTS.  219 

Steps  in  preparation  for  a  more  important  change.     A  time 
comes  when  the  whole  of  the  coverings  of  the  hody  are  at 
once  cast  off,  and  the  insect  assumes  the  form  of  :i  pupa  or 
chrysalis;  being  wrapt  as  in  a  sliroiul,  presenting  no  appear- 
ance of  external  members,  and  retaining  but  feeble  indica- 
tions of  life.     In  this  condition  it  remains  for  a  certain  pe- 
riod:  its  internal  system  continuing  in  secret  the  farther 
consolidation  of  the  organs;  until  the  period  arrives  when 
it  is  qualified  to  emerge  into  the  world,  by  bursting  asun- 
der the  fetters  which  had  confined  it,  and  to  commence  a 
new  career  of  existence.     The  worm,  which  so  lately  crawled 
with  a  slow  and  tedious  pace  along  the  surface  of  the  ground, 
now  ranks  among  the  sportive  inhabitants  of  air;  and  ex- 
panding its  newly  acquired  wings,  launches  forward  into 
the  element  on  which  its  powers  can  be  freely  exerted,  and 
which  is  to  waft  it  to  the  objects  of  its  gratification,  and  to 
new  scenes  of  pleasure  and  delight. 

Thus  do  the  earlier  stages  of  the  development  of  insects 
exhibit  a  recurrence  of  those  structures  which  are  found  in 
the  lowest  department  of  this  series  of  animals.     The  larva, 
or  infantile  stage  of  the  life  of  an  insect,  is,  in  all  its  me- 
chanical relations,  a  mere  worm.     The  imago,  or  perfect 
state,  on  the  other  hand,  exhibits  strong  analogies  with  the 
crustaceous  tribes,  not  only  in  the  general  form  of  the  body, 
but  also  in  the  consolidated  texture  of  its  organs,  (especially 
of  those  which  compose  its  skeleton)  and  in  the  possession 
of  rigid  levers,  shaped  into  articulated  limbs,  and  furnished 
with  large  and  powerful  muscles,  from  all  which  circum- 
stances great  freedom  and  extent  of  motion  are  derived.    To 
this  elaborate  frame,  nature  has  added  wings,  those  refined 
instruments  of  a  higher  order  of  movements,  subservient 
to  a  more  expanded  range  of  existence,  and  entitling  the  be- 
ings on  which  they  have  been  conferred  to  the  most  elevated 
rank  among  the  lesser  inhabitants  of  the  dobe. 

The  mechanical  functions  of  insects  scarcely  admit  of  be- 
ing reduced  to  general  principles,  in  consequence  of  the 
great  diversity  of  forms,  of  habits,  and  of  actions,  that  is  met 


220  THE  MECHANICAL  FUNCTIONS. 

with  among  the  innumcrahle  hosts  of  beings  which  rank  un- 
der this  widely  extended  department  of  the  animal  creation. 
In  these  minute  creatures  may  be  discovered  all  the  me- 
chanical instruments  and  apparatus  required  for  the  execu- 
tion of  those  varied  motions  which  we  witness  in  the  larger 
animals,  and  which,  though  almost  peculiar  to  the  different 
classes  of  these  animals,  are  here  frequently  united  in  the 
same  individual.  Insects  swim,  dive,  creep,  walk,  run,  leap, 
or  fly,  with  as  much  facility  as  fishes,  reptiles,  quadrupeds, 
or  birds.  But  besides  these,  a  great  number  have  also  move- 
ments peculiar  to  themselves,  and  of  v/hich  we  meet  with 
no  example  in  other  parts  of  the  animal  kingdom. 

In  attempting  to  delineate  a  sketch  of  the  movements  of 
insects,  and  of  the  m.echanism  by  which  they  are  performed, 
I  am  compelled,  by  the  great  extent  of  the  subject,  to  confine 
myself  to  very  general  views;  and  must  refer  such  of  my 
readers  as  are  desirous  of  fuller  information  on  this  subject 
to  the  works  of  professed  entomologists. 

The  mechanical  conditions  of  an  insect  in  its  several  states 
of  larva,  pupa,  and  imago,  are  so  widely  different,  that  it 
will  be  necessary  to  consider  each  separately.  In  many  tribes, 
however,  the  difference  between  the  larva  and  the  perfect 
insect  is  much  less  considerable  than  in  others.  Those  be- 
longing to  the  orders  of  Hemiptera  and  Orthoptera,  for  ex- 
ample, come  out  of  the  egg  with  nearly  the  same  form  as 
that  which  they  have  in  the  mature  state;  excepting  that 
they  are  without  wings:  these  organs  being  added  in  the 
progress  of  their  growth,  and  constituting,  when  acquired, 
their  perfect  or  imago  condition. 
« 

§  4.  K/lquatic  Larvx. 

Many  insects,  which,  when  fully  developed,  are  the  most 
perfectly  constructed  for  flying,  are,  when  in  the  state  of 
larvae,  altogether  aquatic  animals.  Some  of  them  are  destitute 
of  feet,  or  other  external  instruments  of  motion,  swimming 
only  by  means  of  the  alternate  inflections  of  the  body  from 
side  to  side,  in  the  same  manner  as  the  Nais,  and  the  Leech. 


AQUATIC  LARViE.  221 

Somollmcs,  these  actions  arc  performed  by  abrupt  strokes, 
giving  rise  to  an  irregular  zig-zag  course:  this  is  the  case 
with  the  larva  of  the  gnat,  and  with  many  others  which  have 
no  feet.  In  the  structure  of  tlie  larva  of  the  Libellula,  or 
dragon-fly,  a  singular  artifice  has  been  resorted  to  for  giving 
an  impulse  to  the  body,  without  the  help  of  external  mem- 
bers. It  is  that  of  the  alternate  absorption  of  water  into  a 
cavity  in  the  hinder  part  of  the  body,  and  its  sudden  ejec- 
tion from  that  cavity,  so  that  the  animal  is  impelled  in  a  con- 
trary direction,  upon  the  same  principle  that  a  rocket  rises 
in  the  air  by  the  reaction  of  that  fluid.  It  has,  at  various 
times,  been  proposed  to  apply  the  power  of  steam  to  the  ])ro- 
duction  of  an  effect  exactly  similar  to  that  of  which  Nature 
here  presents  us  with  so  pcriect  an  example,  for  the  purpose 
of  propelling  ships,  instead  of  the  ordinary  mode  of  steam 
navigation. 

Some  larvae,  such  as  that  of  the  Stratioinys,  collect  a 
bubble  of  air,  which  they  retain  within  a  tuft  of  hair  at  the 
extremity  of  the  tail,  evidently  with  a  view  of  diminishing 
the  specific  gravity  of  the  body,  and  thus  giving  greater  effica- 
cy to  the  muscular  actions  which  they  employ  in  their  pro- 
gression through  the  water.  Another  use  is  also  made  of  these 
tufts  of  hair;  for,  by  repelling  the  water,  they  allow  of  the 
insect's  suspending  itself  from  the  surface  of  the  fluid,  in  the 
manner  already  noticed,  in  giving  the  history  of  the  evolu- 
tions of  the  hydra.* 

The  impulse  given  by  the  lateral  inflections  of  the  body 
are  in  many  cases  assisted  by  short  legs;  but  the  larvae  of 
the  Ephcmer a,  i\\ou^\  furnished  with  legs,  do  not  use  them 
for  this  purpose,  and  swim  simply  by  the  action  of  the  tail. 
Those  of  the  Dyiiscus  are  furnished  with  a  pair  of  very  long 
members,  projecting  to  a  considerable  distance  from  the 
sides,  and  flattened  at  the  ends,  to  serve  as  oars.  The  larvaj 
of  the  Hy  drop  hi  I  us  are  also  admirably  formed  for  swim- 
ming; and  they  not  only  dart  forwards  with  surprising  velo- 
city, but  also  turn  in  all  directions  with  the  utmost  facility. 

•  Page  133. 


222  THE  MECHANICAL  FUNCTIONS. 

§  5»   Terrestrial  Larvae.. 

The  movements  of  larvse  that  are  not  aquatic  are  perfectly 
analogous  to  those  of  the  Annelida,  which  they  much  resem- 
ble in  their  outward  form  and  mechanical  structure.  The 
muscles  by  which  the  annular  segments  of  the  body  are 
moved,  are  exceedingly  numerous,  and  beautifully  arranged 
with  reference  to  the  motions  they  are  intended  to  effect. 
The  investigation  of  the  structure  of  these  minute  organs 
has  Ions  exercised  the  talents  of  the  most  skilful  entomo- 
logists,  and  still  offers  much  that  remains  to  be  explored. 
The  researches  of  Lyonet,  already  alluded  to,  on  the  anato- 
my of  the  larva  of  the  Bomhyx  Cossus,^  of  which  he  has 
published  an  elaborate  description,  accompanied  by  admi- 
rable engravings,  will  ever  remain  a  splendid  monument  of 
patience  and  ingenuity  in  overcoming  the  difficulties  which 
impede  this  kind  of  inquiry.  la  the  body  and  the  limbs  of 
this  caterpillar,  Lyonet  counted  above  4000  separate  muscu- 
lar bands,  all  arranged  with  the  most  perfect  symmetry,  and 
adapted,  with  wonderful  precision,  to  the  performance  of  the 
required  effects. 

In  these  larvse,  as  in  the  simpler  forms  of  the  Annelida, 
progression  is  often  accomplished  solely  by  the  alternate 
contraction  and  extension  of  the  annular  segment,  aided  in 
many  cases,  by  short  hairs,  and  frequently,  also,  by  a  slimy 
secretion  which  exudes  from  their  bodies.  Many  larvae, 
which  are  destitute  of  feet,  move  onwards  by  first  coiling 
the  body  into  a  circle,  making  the  head  and  the  tail  meet,  and 
then  springing  forwards  by  a  sudden  extension  of  the  back, 
producing  an  effect  like  the  unbending  of  a  bow.  By  an 
artifice  of  the  same  kind,  some  larvse  contrive  to  leap  to  a 
considerable  distance,  by  the  violent  efforts  which  they  make 
in  unfolding  the  curvatures  of  their  bodies. 

Some  larvss  avail  themselves  of  their  jaws  in  order  to  fix 
the  head,  and  drag  the  rest  of  the  body  towards  it.  In  this 
manner  do  the  larvse  of  the  Capricorn  beetles  advance  along 

•  C0SSU6  ligniperda.     Fabricius. 


TERRESTRIAL  LAUVJE.  223 

the  winding  passages  which  thc}^  have  themselves  excavated, 
holding  by  the  jaws,  and  dragging  themselves  forwards. 
These  movements  are  assisted  by  the  resistance  aflbrded  by 
short  tubercles  which  project  from  different  parts  of  the  back 
and  under  surface  of  the  body;  so  that  these  insects  advance 
in  the  passage  by  an  act  similar  to  that  by  which  a  chimney- 
sweeper, exerting  the  powerful  pressure  of  his  elbows,  shoul- 
ders, and  knees,  manages  to  climb  up  a  chimney. 

For  the  purpose  of  enabling  insects  to  take  stronger  hold 
of  the  surfaces  they  pass  over,  we  often  observe  them  fur- 
nished with  spines,  or  hooks,  which  are  moved  by  appro- 
priate muscles,  and  they  occupy  different  situations  on  the 
body.  Modifications  without  end  occur  with  regard  to  these 
and  other  external  parts,  subservient  in  various  degrees  to 
progressive  motion.  Every  possible  gradation  is  also  seen 
between  the  short  tubercles  already  mentioned,  and  the  more 
regularly  formed  feet  or  legs.  Those  which  are  regarded  as 
spurious  legs,  or  prolegs,  as  they  have  been  called,  occupy 
an  intermediate  place  between  these  two  extremes.  They 
consist  of  fleshy  and  retractile  tubercles,  and  are  often  very 
numerous;  while  the  number  of  the  l?nie  legs,  as  they  are 
called,  is  limited  to  six.  These  last  are  the  representatives 
of  the  legs  of  the  future  perfect  insect;  for  they  are  attached 
to  the  three  first  segments  of  tlie  thorax;  and  are  formed  of 
those  portions  articulated  to  each  other,  corresponding  to  the 
three  principal  joints  of  the  imago.  The  true  legs  are  gene- 
rally protected  Ijy  horny  scales;  but  the  coverings  of  the  pro- 
legs  are  wholly  membranous.  The  office  of  these  spurious 
legs  is  merely  to  serve  as  props  to  support  tlic  body  while 
the  insect  is  walking,  and  to  prevent  its  hinder  part  from 
trailing  on  the  ground.  They  are  frequently  terminated  by 
single  or  double  hooks;  and  also  by  a  marginal  coronet  of 
recurved  spines.  These  hooks,  or  spines,  enable  the  insect 
to  cling  firmly  to  smooth  surfaces;  and  also  to  grasp  the  most 
slender  twig,  which  could  not  have  been  laid  hold  of  bv  Icirs 
of  the  usual  construction. 

The  speed  with  which  these  larvae  can  advance  is  regu- 
lated by   many   circumstances,   independently  of  the  mere 


224  THE  MECHANICAL    FUNCTIONS. 

possession  of  legs:  for  some  caterpillars  move  slowly,  while 
others  can  run  very  nimbly.  The  following  is  the  order  in 
which  the  legs  are  usually  moved:  namely,  the  anterior  and 
the  posterior  leg  on  the  same  side  are  advanced  at  the  same 
moment,  tosether  with  the  intermediate  one  on  the  other 
side;  and  this  takes  place  alternately  on  both  sides. 

There  is  one  tribe   of  caterpillars  called  Surveyors,  or 
Geometers,  (Fig.  148,*  a)  which  walk  by  first  fixing  the 


fore  feet,  and  then  doubling  the  body  into  a  vertical  arch; 
this  action  brings  up  the  hind  part  of  the  caterpillar,  which 
is  furnished  with  prolegs,  close  to  the  head.  The  hind  ex- 
tremity being  then  fixed  by  means  of  the  prolegs  situated  at 
that  part,  the  body  is  again  extended  into  a  straight  line; 
and  this  process  being  repeated,  the  caterpillar  advances 
by  a  succession  of  paces,  as  if  it  were  measuring  the  distance, 
by  converting  its  body  into  a  pair  of  compasses.  At  the 
same  time  that  they  employ  this  process,  they  farther  pro- 
vide for  their  security  by  spinning  a  thread,  v/hich  they 
fasten  to  different  points  of  the  ground  as  they  go  along. "" 

Many  other  species  of  caterpillar  practise  the  same  art  of 
spinning  fine  silken  threads,  which  especially  assist  them  in 
their  progression  over  smooth  surfaces,  and  also  in  descend- 
ing from  a  height  through  the  air.  The  caterpillar  of  the 
cabbage  butterfly  is  thus  enabled  to  climb  up  and  down  a 
pane  of  glass,  for  which  purpose  it  fixes  the  threads  that  it 
spins  in  a  zig-zag  line,  forming  so  many  steps  of  a  rope  lad- 
der.   The  material  of  which  these  threads  are  made  is  a  glu- 

*  The  great  force  exerted  by  the  muscles  of  many  caterpillars  Is  exempli- 
fied by  their  often  fixing-  themselves  to  an  object,  and  extending  the  body  to 
a  distance,  as  if  it  were  a  rigid  cylinder.  This  attitude  is  shown  in  Fig.  148*  b. 


TREATMENT  OF  LARViE.  225 

tinous  secretion,  which,  on  heing  deposited  on  glass,  adheres 
firmly  to  it,  and  very  soon  acquires  consistence  and  hard- 
ness by  the  action  of  the  air. 

Other  caterpillars  which  feed  on  trees,  and  have  often  oc- 
casion to  descend  from  one  branch  to  another,  send  out  a 
rope  made  with  the  same  material,  which  they  can  prolong 
indefinitely;  and  thus  either  suspend  themselves  at  pleasure 
in  the  air,  or  let  themselves  down  to  the  ground.  They 
continue,  while  walking,  to  spin  a  thread  as  they  advance,  so 
that  they  can  always  easily  retrace  their  steps,  by  gathering 
up  the  clew  they  have  left,  and  reascend  to  the  height  from 
which  they  have  allowed  themselves  to  drop. 

§  6.  Imago,  or  Perfect  Insect, 

The  process  which  nature  has  followed  in  the  develop- 
ment of  the  structure  of  insects,  has  for  its  object  the  gra- 
dual hardening  and  consolidation  of  texture,  and  the  union 
and  concentration  of  organs;  for  we  find  that  the  segments 
which  were  at  a  distance  from  one  another  in  the  larva,  are 
approximated  in  the  perfect  insect,  and  often  closely  tied  to- 
gether by  ligaments:  and  in  other  cases,  adjoining  segments 
cohere  so  as  to  form  but  a  single  piece.  Thus,  the  number  of 
separate  parts  composing  the  solid  fabric  is  considerably  di- 
minished. Other  segments,  again,  fold  inwardly,  forming 
internal  processes,  and  adding  to  the  extent  and  complica- 
tion of  the  skeleton. 

The  integuments  of  perfect  insects,  being  designed  to  be 
permanent  structures,  are  thicker  and  more  rigid  than  those 
of  their  larvae,  and  are  formed  of  several  layers,  in  which 
the  component  parts  of  the  integuments  of  the  larger  ani- 
mals may  readily  be  distinguished.  Their  rigidity  does 
not,  like  that  of  shells,  arise  from  the  presence  of  carbo- 
nate of  lime;  for  they  contain  but  a  small  proportion  of  this 
material:  and  whatever  calcareous  ingredient  enters  into 
their  composition  is  in  the  form  of  phosphate  of  lime.  In 
external  appearance  their  texture  approaches  nearer  to  that 

Vol.  I.  29 


-*«.. 


t« 


226  THE  MECHANICAL  FUNCTIONS.  ~ 

of  horn  than  to  any  other  animal  product:  yet  in  their  che- 
mical composition  they  differ  from  all  the  usual  forms  of  al- 
buminous matter.  The  substance  to  which  they  owe  their 
characteristic  properties  is  of  a  very  peculiar  nature;  it  has 
been  termed  Chiiine  by  M.  Odier,*  and  Entomoline  by 
M.  Lassaigne.t  This  substance  is  found  in  large  quantity  in 
the  wings  and  elytra  of  coleopterous  insects.  It  is  remark- 
able for  not  liquefying,  as  horn  does,  by  the  action  of  heat; 
and  accordingly  the  integuments  of  insects,  even  after  having 
been  subjected  to  a  red  heat,  and  reduced  to  a  cinder,  are 
found  to  retain  their  original  form.:}: 

With  this  substance  there  is  blended  a  quantity  of  colour- 
ing matter,  which  has  usually  a  dull  brown  or  black  hue. 
But  the  colour  of  the  external  surface  is  generally  owing  to 
another  portion  of  this  matter,  which  is  spread  over  it  like 
a  varnish,  and  being  soluble  in  alcohol  and  in  ether,  may  be 
removed  by  means  of  these  agents.  The  colours  which  are 
displayed  by  insects,  and  which  arise  from  the  presence  of 
this  latter  substance,  are  often  very  brilliant,  and,  as  is  the 
case  with  many  other  classes  of  animals,  the  intensity  of  the 
tints  is  heightened  by  the  action  of  light.  The  elytra  of 
tropical  insects  display  a  gorgeous  metallic  lustre  depending 
on  the  reflection  of  the  prismatic  colours;  and  the  same  va- 
riegated hues  adorn  the  scales  of  butterflies  of  those  regions. 

Hair  grows  in  various  parts  of  the  surface  of  insects. 
Where  the  integument  is  membranous  and  transparent,  these 
hairs  may  be  distinctly  perceived  to  originate  from  enlarged 
roots,  or  bulbs,  and  to  pass  out  through  apertures  in  the  skin; 
as  is  the  case  with  the  hair  of  the  larger  animals.  Their 
chemical  composition,  however,  is  very  different,  for  they 
are  formed  of  the  same  substance  as  the  integuments,  name- 

•  Annales  de  Chimle,  torn.  76. 

I  See  the  work  of  Straus  Durckheim,  p.  33. 

t  M.  Odier  had  concluded  from  his  experiments  that  no  nitrog-en  enters 
into  the  composition  of  this  substance.  That  this  conclusion  has  been  too 
hastily  adopted  has  been  proved  by  Mr.  Children,  who,  by  pursuing"  another 
mode  of  analysis,  found  that  the  chitlne  of  cantharides  contains  not  less  than 
nine  or  ten  per  cent,  of  nitrogen.     See  Zoological  Journal,  i.  Ill — 115. 


STRUCTURE  OF  INSECTS. 


227 


ly,  entomoline.  The  purposes  served  by  the  hairs  are  not 
always  obvious.  In  many  cases  they  seem  intended  to  pro- 
tect the  integuments  from  the  water,  which  they  repel  from 
their  surfaces.  They  also  tend  to  prevent  injury  arising 
from  friction;  and  are  found  to  be  more  abundant  in  those 
parts,  as  the  joints,  vvhich  are  liable  to  rub  much  against  one 
another. 

The  divisions  of  the  body  are  frequently  marked  by  deep 
incisions;  whence  has  originated  the  term  insect,  expressive 
of  this  separation  into  sections.  It  is,  however,  a  character 
which  they  possess  in  common  with  all  articulated  animals, 
the  typical  form  of  which  consists,  as  we  have  seen,  of  a  se- 
ries of  rings,  or  segments,  joined  endwise  in  the  direction  of 
a  longitudinal  axis.     The  principal  portions  into  which  the 

body  is  divided  are  the  heady 
the  trunk,  and  the  abdomen: 
each  of  which  is  composed  of 
several  segments.   I  have  here 
given,  in  illustration,  the  an- 
nexed figures,showing  the  suc- 
cessive   portions    into    which 
the  solid  frame-work,  or  ske- 
leton,  of  one    of  the    beetle 
tribe,    the     Calosoma    syco- 
phanta,"*  may  be  separated. 
The  entire  insect,  which  pre- 
sents the  most  perfect  speci- 
men of  a  complete  skeleton  in  this  class  of  animals,  is  repre- 
sented in  Fig.  149;  and  the  several  detached  segments,  on 
an  enlarged  scale,  in  Fig.  150.     The  head  c,  as  seen  in  the 
latter  figure,  may  be  regarded  as  being  composed  of  three 
segments:  the  trunk,  x,  y,  z,  of  three;  and  the  abdomen,  b, 
of  nine.     Fig.  151,  is  a  view  of  the  head  separated  from  the 
trunk,  and  seen  from  behind,  in  order  to  show  that  its  form 
is  essentially  annular,  and  that  it  resembles  in  this  respect 
the  rings  of  which  the  thorax  consists,  and  to  which  it  forms 
a  natural  sequel. 


■4>. 


•  Carnhus  sycophanta.     Linn. 


22S 


THE  MECHANICAL  FUNCTIONS. 


The  head  contains  the  brain,  or  principal  enlargement  of 
the  nervous  system,  and  the  organs  of  sensation  and  of  mas- 
tication.    Its  size,  as  compared  with  the  rest  of  the  body, 


varies  much  in  different  insects,  and  is  in  general  propor- 
tionably  larger  than  it  is  in  the  larva  state.  Its  integument, 
which,  from  analogy  with  vertebrated  animals,  has  been 
called  the  skull,  or  cranmm,  (c,  Fig.  150,)  is  usually  the 
hardest  part  of  the  general  crust.  Although  it  may  appear, 
on  a  superficial  examination,  to  consist  of  a  single  undivided 
piece,  yet,  on  tracing  its  gradual  formation,  it  is  found  to  be 
in  reality  composed  of  a  union  of  several  of  the  segments  of 
the  larva.     Audouin  and  Carus  distinguish  three  component 


STRUCTURE  OF  INSECTS.  Qoq 

segments  in  the  cranium  of  insects;  while  Straus  Durckhcim 
considers  it  as  formed  by  the  consolidation  of  no  less  than 
six  segments  of  the  vermiform  larva.  According  to  this 
theory,  the  same  elements  which  in  the  thoracic  segments 
are  developed  into  feet,  are  here  employed  to  form  parts 
having  other  destinations.  From  the  segment  adjacent  to 
the  thorax  the  antennae  are  supposed  to  be  developed.  The 
two  anterior  segments  belong  properly  to  the  face;  the  one 
giving  origin  to  the  mandibles,  (m,)  to  the  maxillae,  or  proper 
jaws,  (j,)  and  also  to  the  palpi,  (p;)  the  other  producing  the 
processes  called  the  labial  palpi,  (l.  ) 

The  mode  in  which  the  head  is  connected  with  the  trunk 
varies  much  in  different  insects.  Sometimes  it  is  united  by 
a  broad  basis  of  attachment,  forming  a  joint  between  the  ad- 
jacent surfaces:  but  usually  it  is  only  appended  by  a  narrow 
filament,  or  neck;  so  that  the  articulation  is  effected  by  liga- 
ment alone.  Occasionally,  it  is  placed  at  the  end  of  a  long 
pedicle,  which  removes  it  to  a  considerable  distance  from 
the  trunk.  In  the  HymenojHera  and  Diptera^  the  head 
moves  upon  a  pivot,  so  as  to  admit  of  its  being  turned  com- 
pletely round. 

The  trunk,  or  Thorax^  is  composed,  as  shown  in  the  figure, 
of  three  segments,  termed  respectively  the  Prothorax  (x;) 
the  Mesothorax  (y;)  and  the  Meiaihorax  (z.*)  The  first 
of  these,  the  prothorax,  carries  the  first  pair  of  legs:  the  se- 
cond, or  mesothorax,  gives  origin  to  the  second  pair  of  legs, 
and  also  to  the  first  pair  of  wings,  or  to  the  Elytra  (e,)  as  ia 
the  example  before  us;  and  the  third,  or  metathorax,  supports 
the  third  pair  of  legs,  and  the  second  pair  of  wings  w.) 
These  two  last  segments  are  closely  united  together,  but 
the  original  distinction  into  two  portions  is  marked  by  a 

*  In  these  denominations  I  have  followed  the  nomenclature  of  Victor  Au- 
douin  (Annales  des  Sciences  Naturelles,  torn.  i.  p.  119,)  as  being-  the  sim- 
plest and  clearest:  but  other  entomolog-ists  have  applied  the  same  terms  to 
different  parts.  The  first  segment  is  termed  by  Straus  Durckheim  and 
other  French  writers,  the  Corselet.  Mr.  Kirby  calls  it  the  Manitrunk,  and 
restricts  the  term  Prothorax  to  its  upper  portion.  The  united  second  and 
third  se^ents  are  the  Thorax  of  Straus  Uurckheim,  the  Tronc  alt/ere  of 
Chambrier,  and  the  Mitrunk  of  Kirby. 


230  THE  MECHANICAL  FUNCTIONS. 

transverse  line.  Each  of  these  three  segments  is  divisible 
into  an  upper,  a  lower  and  two  lateral  portions,  which  are 
joined  together  at  the  sides  of  the  trunk;  these  again  admit 
of  farther  subdivision;  but  for  the  names  and  descriptions  of 
these  smaller  pieces  I  must  refer  the  reader  to  works  on 
Entomology.  The  parts  of  the  thorax  to  which  the  wings 
are  attached  indicate  the  situation  of  the  centre  of  gravity  of 
the  whole  insect;  a  point  which  being  in  the  line  of  the  re- 
sultant of  all  the  forces  concerned  in  the  great  movements 
of  the  bod}^,  requires  to  be  sustained  by  the  moving  powers 
under  all  circumstances  either  of  action  or  repose. 

Victor  Audouin,  who  has  made  extensive  researches  on 
the  comparative  forms  of  all  these  parts  in  a  great  variety 
of  insects,  appears  to  have  satisfactorily  established  the  ge- 
neral proposition  that,  amidst  the  endless  diversity  of  forms 
exhibited  by  the  skeleton  of  insects,  they  are  invariably 
composed  of  the  same  number  of  elements,  disposed  in  the 
same  relative  situations  and  order  of  arrangement:  and  that 
the  only  source  of  difference  is  a  variation  in  the  propor- 
tional development  of  these  elements.  He  has  also  observed 
•  that  the  great  expansion  of  one  part  is  generally  attended 
by  a  corresponding  diminution  of  others. 

The  third  division  of  the  body  is  termed  the  Mdomen 
(b;)  it  is  composed  of  all  the  remaining  segments,  which  join 
to  form  a  cavity  enclosing  the  viscera  subservient  to  nutri- 
tion, respiration,  and  reproduction.  The  number  of  these 
abdominal  segments  is  very  various  in  different  genera  of 
insects.  Sometimes  there  appear  to  be  but  three  or  four; 
while,  in  other  cases,  there  are  twelve  or  even  a  greater 
number.  In  the  Calosoma  (Fig.  150,  b,)  the  abdomen  has 
six  complete,  followed  by  three  imperfect  segments.  Not 
being  intended  to  carry  any  of  the  organs  of  progressive 
motion,  they  retain  the  form  of  single  hoops,  which  is  the 
primitive  type  of  the  segments  of  annulose  animals.  Each 
segment  has  a  ligamentous  connexion  with  the  next,  which 
is  often  so  close,  as  hardly  to  admit  of  any  motion  between 
them;  but  in  other  instances  It  is  more  lax,  and  allows  of 
the  abdomen  being  flexible.   In  the  former  case,  which  is  the 


STRUCTURE  OF  INSECTS.  231 

construction  in  all  the  Coleopiera,  or  heeilcs,  the  rings  have 
an  imbricated  arrangement;  that  is,  each  overlap  the  next, 
often  to  the  extent  of  two-thirds  of  its  breadth:  so  that  they 
present  a  succession  of  spheroidal  hoops,  capable  of  beino- 
drawn  out,  to  a  certain  extent,  like  the  tubes  of  a  telescope. 
This  very  artificial  construction  is  manifestly  designed  to 
allow  of  a  great  variety  of  movements,  determined  by  the 
position  of  the  muscles  they  enclose:  for  since  the  surfaces 
which  receive,  as  well  as  those  which  are  received,  are  seg- 
ments of  spheroids,  this  structure  admits  of  a  twistino-  mo- 
tion;  and  the  latter  segment  may  be  pushed  more  or  less 
into  the  cavity  of  the  former,  either  generally,  or  on  one 
side. 

Each  segment,  besides  being  separate  from  the  rest,  is  far- 
ther divided  into  an  upper,  or  dorsal,  and  a  lower,  or  ventral 
portion;  each  portion  having  the  form  of  a  semicircle,  or  ra- 
ther of  an  arch  of  a  circle.  These  are  connected  at  the  sides 
by  a  ligamentous  band,  which  runs  the  whole  length  of  the 
abdomen.  Great  advantage  results  from  this  division  of  the 
circles,  allowing  of  the  upper  and  lower  portions  of  the  ab- 
dominal covering  being  at  one  time  separated,  and  at  ano- 
ther brought  nearer  together;  for  thus  the  cavity  is  capable 
of  being  enlarged  or  contracted  in  its  dimensions,  and  adapt- 
ed to  the  variable  bulk  of  its  contents.  It  is  deservino-  of 
notice  that,  during  the  process  of  transformation,  some  of 
the  abdominal  segments,  which  are  present  in  the  larva,  dis- 
appear entirely,  or  leave  only  imperfect  traces  of  their  for- 
mer existence.  Sometimes  the  posterior  segments  become 
so  exceedingly  contracted  in  their  diameter  as  to  give  rise 
to  the  appearance  of  a  tail:  this  is  exemplified  in  the  Pa- 
Tiorpa. 

The  junction  of  the  abdomen  with  the  trunk  is  effected 
in  various  ways.  In  all  the  Coleoptera,  it  is  united  by  the 
whole  margin  of  its  base,  without  having  a  narrower  part: 
in  other  tribes  there  is  a  visible  diminution  of  diameter,  form- 
ing a  groove  all  round,  or  an  incision^  as  it  is  technically 
termed.  In  the  Ilymenoptera,  this  incision  is  so  deep  as  to 
leave  only  a  narrow  pedicle,  like  a  neck,  connecting  these 


232  THE  MECHANICAL  FUNCTIONS. 

two  divisions  of  the  body.  In  some  this  pedicle  is  short,  in 
others  long:  in  the  former  case,  an  exceedingly  refined  me- 
chanism is  resorted  to  for  effecting  the  necessary  movements 
in  a  part  so  bulky  compared  with  the  narrowness  of  the  sur- 
face of  attachment* 

Insects  in  their  perfect  state  have  constantly  six  legs, 
which  are  the  developments  of  the  six  proper  legs  of  the 
same  animal  in  its  larva  condition:  all  the  spurious  legs 
having  disappeared  during  its  metamorphosis.  We  have 
seen  that  in  the  myriapoda,  the  result  of  development  is  an 
increase  in  the  number  both  of  segments  and  of  legs;  the 
reason  of  which  is  that,  being  terrestrial  animals,  a  length- 
ened form  was  more  useful  and  accordant  with  their  desti- 
nation; but  in  winged  insects,  where  the  object  is  to  procure 
the  means  of  flight,  the  organs  require  to  be  concentrated, 
and  all  superfluous  parts  must  be  retrenched  and  discarded 
from  the  fabric.  The  multiplication  of  organs,  which,  in  the 
former  case,  indicated  the  progress  of  a  higher  development, 
would  in  the  latter  have  been  the  source  of  imperfection. 
As  long  as  the  insect  remains  in  its  larva  stage,  its  condition 
is  analogous  to  that  of  the  myriapode:  but  in  the  more  ele- 
vated state  of  its  existence,  its  structure  is  subject  to  new 
conditions  and  regulated  by  new  laws. 

While  the  number  of  members  is  thus  reduced,  ample 
compensation  is  given  by  their  increased  activity  and  power, 
derived  from  their  augmented  length,  and  the  more  distinct 
lever-like  forms  of  the  pieces  which  compose  them. 

These  pieces  (see  Fig.  150)  are  named,  from  their  sup- 
posed analogy  to  the  divisions  of  the  limbs  of  the  higher  or- 
ders of  vertebrated  animals,  the  haunch  (h,)  the  trochanter 
(t,)  the  femur  (f,)  the  tibia  (s,)  and  the  tarsus  (r.)  In  ge- 
neral the  femur  (or  thigh)  has  nearly  a  horizontal,  and  the 
tibia  (or  leg)  a  vertical  position,  while  the  whole  tarsus  (or 
foot)  is  applied  to  the  ground. 

The  haunch  (h,)  which  is  supposed  to  correspond  to  the 

*  For  the  details  of  this  structure  I  must  refer  to  writers  on  entomology, 
and  in  particular  to  Kirby  and  Spence's  « *  Inti-oduction  to  Entomology," 
vol.  iii.  p.  701. 


STRUCTURE  OP  INSECTS.  033 

hip  bone  of  quadrupeds,  is  a  broad,  but  very  short  truncated 
cone.     The  mode  of  its  articulation  with  the  trunk  admits 
of  great  variety;  sometimes  it  is  united  by  a  ball  and  socket 
joint,  as  in  the  CurciiUo  and  Ccramhyx;  and  it  then  has,  of 
course,  great  freedom  of  motion:  at  other  times  the  joint  is 
of  the  hinge  kind,  as  in  the  Mdolontha.     The  trochanter 
(t,)  and  the  femur  (f,)  though  in  reality  distinct  pieces,  are 
usually  so  firmly  united  as  to  compose  only  one  division  of 
the  limb.     The  articulation  of  this  portion  with  the  haunch 
is  always  effected  by  a  hinge-joint.     Joints  of  this  descrip- 
tion, when  formed,  as  they  are  in  insects,  by  the  apposition 
of  two  tubular  pieces,  are  constructed  in  the  following  man- 
ner.    One  of  the  tubes  has,  at  the  end  to  be  articulated,  two 
tubercles,  which  project  from  the  margin,  and  are  applied  to 
the  adjacent  end  of  the  other  tube  at  two  opposite  points  of 
its  circumference;  the  line  which  passes  through  those  two 
points  being  the  axis  of  motion.     On  the  side  where  the 
flexion  is  intended  to  be  made  both  tubes  are  deeply  notched, 
in  order  to  admit  of  their  being  bent  upon  one  another  at  a 
very  acute  angle:  and  the  space  left  by  these  notches  is  filled 
up  by  a  pliant  membrane,  which  performs  the  office  of  a  li- 
gament.    These  articular  tubercles  and   dejnessions  are  so 
adjusted  to  one  another,  that  the  joint  cannot  be  dislocated 
without  the  fracture  of  some  of  its  parts.      As  the  different 
axes  of  motion  in  the  successive  joints  are  not  coincident, 
but  inclined  at  different  angles  to  one  another,  the  extent  of 
motion  in  the  whole  limb  is  very  greatly  increased.    Thus, 
in  the  cases  where  the  articulation  of  the  haunch  with  the 
trunk  is  a  hinge-joint,  the  axes  of  this  ioint  and  of  the  next 
are  placed  at  right  angles  to  each  other;  so  that  there  results, 
from   the  combination  of  both,  a  caj)ability  in   the  thigh  of 
executing  a  circular  motion  in  a  manner  almost  as  perfect 
as  if  it  had  revolved  in  a  spherical   socket.     The  principle 
of  this  compound   motion  is  the  same  as  that  employed  on 
ship-board  for  the  mariner's  compass,  and  other  instruments 
which  require  to  be  kept  steady  during  the  motion  of  the 
ship.     For  this  purpose  what  arc  called  gimhah  arc  used, 
the  parts  of  which  have  two  axes  of  rotation,  at  right  angles 
Vol.  I.  30 


234  THE  MECHANICAL  FUNCTIONS. 

to  each  other,  so  as  to  enable  the  compass  to  talie  its  proper 
horizontal  position,  independently  of  any  inclination  of  the 
ship. 

The  tibia,  or  shank  (s,)  is  joined  at  an  acute  angle  with 
the  femur;  and  is  frequently  either  beset  with  spines,  or  else 
notched  or  serrated. 

The  tarsus,  or  foot  (r,)  is  the  last  division  of  the  limb:  it 
is  divided  into  several  joints,  which  have  been  supposed  to 
represent  those  of  the  toes  of  quadrupeds.  The  joints  are 
generally  of  the  hinge  kind,  but  some  are  met  with  of  a 
more  rounded  form,  and  approaching  to  that  of  the  ball  and 
socket.  The  whole  structure  is  most  admirably  adapted  to 
its  exact  application  over  all  the  inequalities  of  the  surfaces 
on  which  the  insect  treads.  But  as  the  habits  and  modes  of 
life  of  this  numerous  class  are  exceedingly  diversified,  so 
the  form  of  the  feet  admits  of  greater  variety  than  that  of 
any  other  part  of  the  limb. 

The  feet  of  insects  diverge,  and  spread  over  a  wide  sur- 
face; thus  extending  the  base  of  support  so  as  to  ensure  the 
stability  of  their  bodies  in  the  most  perfect  manner.  When 
the  legs  are  very  long,  as  in  the  Tipula,^^  the  body  seems, 
indeed,  more  to  be  suspended  than  supported  by  them;  con- 
trary to  what  obtains  in  quadrupeds,  where  the  feet  are 
more  immediately  underneath  the  points  at  which  they  are 
connected  with  the  trunk. 

The  last  joint  of  the  tarsus  is  generally  terminated  by  a 
claw,  which  is  sometimes  single  and  sometim.es  double,  and 
which  contributes  to  fasten  the  foot,  under  a  variety  of  cir- 
cumstances, both  of  action  and  of  repose.  With  feet  thus 
armed,  the  insect  can  ascend  or  descend  the  perpendicular 
sides  of  a  rough  body  with  the  greatest  ease;  but  it  is  scarce- 
ly able  to  advance  a  single  step  upon  glass,  or  other  polished 
surfaces,  even  when  horizontal.  The  hooks  at  the  ends  of 
the  anterior  pair  of  feet  are  directed  backwards,  those  of  the 
middle  pair  inwards,  and  of  the  posterior  pair  forwards;  thus 
afibrding  the  greatest  possible  securit}^  against  displacement. 

•  It  has  been  conjectured  that  the  object  In  furnishing-  this  insect  with  leg's 
of  so  great  a  length  is  that  of  enabling  it  to  walk  among-  blades  of  g-rass. 


STRUCTURE  OF  INSECTS. 


235 


Many  insects  are  provided  with  cushions  at  the  extremity 
of  the  feet,  evidently  for  the  purpose  of  breaking  the  force 
of  falls,  and  preventing  the  jar  which  the  frame  would  other- 
wise have  to  sustain.     These  cushions  arc  formed  of  dense 
velvety  tufts  of  hair,  lining  the  underside  of  the  tarsi,  but 
leaving  the  claw  uncovered;  and  the  filaments,  by  insinuat- 
ing themselves  among  the  irregularities  of  the  surfaces  to 
which  they  are  applied,  produce  a  considerable  degree  of  ad- 
hesion.     Cushions  are  met  with  chiefly  in  large  insects 
which  suddenly  alight  on  the  ground  after  having  leaped 
from  a  considerable  height:  in  the  smaller  species  they  ap- 
pear to  be  unnecessary,  because  the  lightness  of  their  bodies 
sufficiently  secures  them  from  any  danger  arising  from  falls. 
Some  insects  are  furnished  with  a  still  more  refined  and 
effectual  apparatus  for  adhesion,  and  one  which  even  enables 
them  to  suspend  themselves  in  an  inverted  position  from  the 
under  surfaces  of  the  bodies.     It  consists  of  suckers,  the  ar- 
rangement and  construction  of  which  are  exceedingly  beau- 
tiful; and  of  which  the  common  house-fly  presents  us  with 
an  example.     In  this  insect  that  part  of  the  last  joint  of  the 
tarsus  which  is  immediately  under  the  root  of  the  claw,  has 
two  suckers  appended  to  it  by  a  narrow  funnel-shaped  neck, 
moveable  by  muscles  in  all  directions.     These  suckers  are 
shown  in  Fig.  152,  which  represents  the  under  side  of  tbe 
foot  oiMusca  vomitoria^  or  blue-bottle  fly,  with  the  suckers 
expanded.     The  sucking  part  of  the  apparatus  consists  of  a 
membrane,  capable  of  contraction  and  extension,  and  the 
edges  of  which  are  serrated,  so  as  to  fit  them  for  the  closest 
application  to  any   kind  of  surface.     In  the   Tahcmiis,  or 


horse-fly,   each  foot  is  furnished  with    three  suckers.     In 
the  Cimbex  hUea,  or  yellow  saw-fly,  there  are  four,  of  which 


236  THE  MECHANICAL  FUNCTIONS. 

one  is  placed  upon  the  under  surface  of  each  of  the  four  first 
joints  of  the  toes,  (Fig.  153;)  and  all  the  six  feet  are  pro- 
vided with  these  suckers.  In  the  Dyiisciis  niarginaliSy 
suckers  arc  furnished  to  the  feet  of  the  male  insect  only. 
The  three  first  joints  of  the  feet  of  the  fore-legs  of  that  in- 
sect have  the  form  of  a  shield,  the  under  surface  of  which  is 
covered  with  suckers  having  long  tubular  necks:  there  is 
one  of  these  suckers  very  large,  another  of  a  smaller  size, 
and  a  great  number  of  others  exceedingly  small.  A  few  of 
the  latter  kind  are  represented  highly  magnified  in  Fig.  154. 
In  the  second  pair  of  feet,  the  corresponding  joints  are  pro- 
portionally much  narrower,  and  are  covered  on  their  under 
surface  with  a  multitude  of  very  minute  suckers.  The  Jicri- 
diinn  biguttiihivi,  wdiich  is  a  species  of  grasshopper,  has 
one  large  oval  sucker,  under  the  last  joint  of  the  foot,  im- 
mediately between  the  claws.  On  the  under  surface  of  the 
first  joint  are  three  pairs  of  globular  cushions,  and  another 
pair  under  the  second  joint.  Fig.  155  shows  these  parts. 
The  cushions  are  filled  with  an  elastic  fibrous  substance; 
which,  in  order  to  increase  the  elasticity  of  the  whole  struc- 
ture, is  looser  in  its  texture  towards  the  circumference.^ 

The  mode  in  which  these  suckers  operate  may  be  dis- 
tinctly seen,  by  observing  with  a  magnifying  glass  the  ac- 
tions of  a  large  blue-bottle  fly  in  the  inside  of  a  glass  tum- 
bler. A  fly  will,  by  the  application  of  this  apparatus,  remain 
suspended  from  the  ceiling  for  any  length  of  time  without  the 
least  exertion;  for  the  weight  of  the  body  pulling  against 
the  suckers  serves  but  to  strengthen  their  adhesion:  hence, 
we  find  flies  preferring  the  ceiling  to  the  floor,  as  a  place  of 
rest. 

Insects  which,  like  the  gnat,  walk  much  upon  the  surface 
of  water,  have  at  the  ends  of  their  feet  a  brush  of  fine  hair, 
the  dry  points  of  which  appear  to  repel  the  fluid,  and  pre- 
vent the  leg  from  being  wetted.  If  these  brushes  be  moist- 
ened with  spirit  of  wine,  this  apparent  repulsion  no  longer 
takes  place;  and  the  insect  immediately  sinks  and  is  drowned. 

*  Philosophical  Transactions  for  1S26,  p.  324. 


AQUATIC  INSECTS. 


237 


§  7.  */2quatic  Insects. 

Although  many  insects  arc  inhabitants  of  water  while 
in  their  larva  state,  few  continue  to  reside  in  that  element 
after  they  have  undergone  all  their  metamorphoses.     When 
they  have  attained  the  imago  state,  indeed,  every  part  of 
their  bodies  becomes  permeated  by  air,  which  forms  alto- 
gether a  large  portion  of  their  bulk,  and  gives  to  the  insect, 
when  it  is  immersed  in  water,  a  strong  buoyant  force.     As 
the  largest  volume  of  air  is  contained  in  the  abdomen,  this 
part  is  comparatively  lighter  than  either  the  trunk  or  head; 
and  the  natural  position  of  the  insect  in  the  fluid  is  oblique' 
to  the  horizon,  the  head  being  depressed,  and  the  abdomen 
elevated.     Any  force  impelling  the  body  forwards  in  the 
direction  of  its  axis  tends,  therefore,  to  make  italso  descend. 
The  effect  of  this  downward  force  is  counteracted  by  the 
sustaining  pressure  of  the  water,  which  is  directed  vertically 
upwards:  so  that  the  real  operation  of  the  force  in  question 
is  to  carry  the  body  forwards  nearly  in  a  horizontal  di- 
rection. 

In  insects  destined  to  move  in  water,  sometimes  all  the 
legs,  but  occasionally  only  one  pair,  are  lengthened  and  ex- 
panded into  broad  triangular  surilices,  capable  of  acting  as 


oars:  and  these  surfaces  arc  farther  extended  by  the  addi- 
tion of  marginal  fringes  of  hair,  so  disposed 'as  to  project 
and  act  upon  the  water  every  time  the  im])ulsc  is  given,  but 
to  bend  down  when  the  leg  is  again  drawn  up,  prci)aratory 


23S  THE   MECHANICAL  FUNCTIONS. 

to  the  succeeding  stroke;  thus  imitating  the  action  which  is 
called  feathering  an  oar.  The  impulses  are  given  with  great 
regularity,  all  the  feet  striking  the  water  at  the  same  mo- 
ment. 

Of  all  the  coleopterous  insects,  the  Dytiscus,  or  water- 
beetle  (of  which  Fig.  156  represents  the  upper,  and  Fig.  157 
the  under  side,)  is  the  one  best  constructed  for  swimming: 
its  body  having  a  flattened  form,  very  much  resembling  a 
boat,  narrower  before  than  behind,  and  its  surface  present- 
ing no  projecting  parts.  The  upper  surface  in  particular  is 
extremely  smooth,  to  enable  it  to  glide  under  the  water  with 
the  least  possible  friction.  Its  centre  of  gravity  is  placed 
very  near  the  under  surface.  The  posterior  legs,  which  act 
as  powerful  oars,  are  attached  to  very  large  haunches,  for 
the  purpose  of  containing  the  thick  muscular  bands  which 
are  inserted  into  the  trochanter,  and  by  which  these  joints 
are  moved  with  great  power.  As  the  motion  of  these  oars 
is  to  be  performed  in  a  plane  nearly  parallel  to  the  axis  of 
the  body,  the  haunches  are  not  required  to  be  moveable: 
and  accordingly  they  are  firmly  united  to  the  thorax;  a 
structure  which  renders  the  motion  of  the  other  joints  more 
regular  and  uniform.  When  the  Dytiscus  wishes  to  rise,  it 
need  only  desist  from  all  action,  and  abandon  itself  to  the 
buoyant  force  of  the  fluid,  which  quickly  carries  it  to  the 
surface. 

The  Notonecta^  or  water-boatman  (Fig.  15S,)  is  remarka- 
ble for  always  swimming  on  its  back,  a  peculiarity  depend- 
jrg  ing  on  the  form  of  its  body,  which  is- 

semi-cylindrical,  with  the  legs  aflixed  to 
the  flat  surface;  so  that,  when  lying  on 
its  back  in  the  fluid,  the  centre  of  gravity 
is  below  the  centre  of  the  whole  figure, 
or  the  metacentre,  as  it  is  termed,  and 
the  equilibrium  is  maintained.  It  is  evident  that,  under  these 
circumstances,  if  it  were  placed  in  the  water  with  its  legs 
undermost,  it  *would  unavoidably  tilt  over,  and  resume  its 
usual  position.  Its  long  legs  extending  at  right  angles  to 
the  body,  present  a  striking  resemblance  to  the  oars  of  a  boat; 


PROGRESSIVE  MOTION  OF  INSECTS.  039 

and  they  act,  indeed,  in  the  same  manner,  and  on  the  same 
prineii:)les. 

§  8.  Progressive  Motion  of  Insects  on  Land, 

The  actions  of  the  limbs  of  insects  in  walking  are  quite 
different  from  what  they  are  in  swimming,  and  are  very 
similar  to  those  of  the  caterpillar,  in  which  we  have  seen 
that  the  motions  of  the  anterior  and  posterior  legs  on  one 
side  are  combined  with  that  of  the  middle  one  on  the  other 
side;  and  the  two  sets  of  legs  arc  moved  alternately.  In 
consequence  of  their  relative  positions  with  the  trunk,  the 
anterior  legs  are  advanced  by  the  extension,  and  the  poste- 
rior legs  by  the  flexion  of  the  corresponding  joints.  When 
the  feet  have  fixed  themselves  on  the  ground,  the  contra- 
ry actions  take  place,  and  the  body  is  brought  forwards. 
During  this  period  the  legs  which  compose  the  other  set  arc^ 
called  into  play,  and  are  advanced;  and  the  same  succession 
of  actions  takes  place  with  these  as  with  the  former.  This 
can  easily  be  seen  when  the  insect  walks  very  leisurely;  but 
in  a  more  quickened  pace,  the  succession  of  actions  is  too 
rapid  to  be  followed  by  the  eye. 

The  action  of  leaping  is  performed  by  the  sudden  exten- 
sion of  all  the  joints  of  the  limb,  which  are  prcviouslv 
folded  as  close  as  possible.  The  joints  principally  concerned 
in  this  action,  are  those  of  the  tliigli  and  tibia,  as  they  fur- 
nish the  longest  and  most  powerful  levers.  Preparatory 
to  the  effort,  the  tibia  is  brought  down  as  close  as  possible 
to  the  ground,  by  bending  it  over  the  tarsus;  and  the  thigh 
also  is  bent  upon  the  tibia,  so  as  to  form  with  it  a  very  acute 
angle.  In  order  to  enable  it  to  take  this  position  with  most 
advantage,  we  find  In  many  of  the  Coleoptcra,  that  the  thigh 
has  a  longitudinal  groove  for  the  reception  of  the  tibia,  with 
a  row  of  spines  on  each  side  of  the  groove.  While  the 
limb  is  in  this  bent  position,  the  extensor  muscles  are  vio- 
lently exerted,  and  by  producing  a  sudden  unbending  of  this 
apparatus  of  folded  springs,  they  project  tlie  whole  body, 


^40  THE  MECHANICAL  FUNCTIONS. 

by  the  accumulated  impulse,  to  a  considerable  height  in  the 
air.  The  leaps  of  insects  being  generally  forwards,  all  the 
legs  do  not  participate  equally  in  the  effect;  for  the  fore  legs 
contribute  much  less  to  it  than  the  hind  legs,  and  are  more 
useful  in  modifying  the  direction  of  the  leap,  than  in  adding 
to  its  force.  The  power  of  leaping  is  derived  principally 
from  the  great  size  and  strength  of  the  extensor  muscles  of 
the  legs,  which,  being  contained  within  the  femur,  necessa- 
rily swell  that  division  of  the  limb  to  an  unusual  thickness; 
and  in  order  to  procure  sufficient  velocity  of  action,  both  the 
femur  and  tibia  are  much  elongated.  Thus,  the  locust,  which 
is  so  constructed,  leaps  with  ease  to  a  distance  two  hundred 
times  the  length  of  its  own  body.  We  may  in  general,  in- 
deed, infer  the  particular  kind  of  progressive  motion  for 
which  the  insect  is  intended  by  observing  the  comparative 
length  of  the  different  pairs  of  legs.  When  they  are  of  equal 
size,  the  pace  is  uniform:— swiftest  in  those  that  have  the 
#  longest  legs, — slowest  when  they  are  short.  When  the  an- 
terior legs  are  much  longer  than  the  posterior,  the  power  of 
prehension  may  be  increased,  but  that  of  progression  is  im- 
peded. The  great  prolongation  of  the  posterior  legs  is  ge- 
nerally accompanied  by  the  power  of  jumping,  unless,  in- 
deed, they  are  at  the  same  time  much  bent,  for  such  curvature 
disqualifies  them  from  acting  advantageously  as  levers. 

Many  insects  have  the  extremity  of  the  tibia  armed  with 
a  coronet  of  spines,  which  assist  in  fixing  this  point  against 
the  plane  from  w^hich  they  intend  to  spring,  and  which  give 
to  the  limb  a  steady  fulcrum.  The  Cicada  spitmaria  has 
been  known  to  leap  to  a  distance  of  five  or  six  feet;  which 
is  two  hundred  and  fifty  times  its  own  length:  this,  if  the 
same  proportions  were  observed,  is  equivalent  to  a  man  of 
ordinary  stature  vaulting  through  the  air  the  length  of  a 
quarter  of  a  mile.  When  the  same  insect  is  laid  on  glass, 
on  which  the  spines  cannot  fasten,  it  is  unable  to  leap  far- 
ther than  six  inches.* 

The  insects  belonging  to  the  genus  Elciier  are  provided 

•  De  Geer,  III.  178,  quoted  by  Kii-by  and  Spence. 


PROGRESSIVE  MOTION  OP  INSECTS.  241 

with  a  peculiar  mechanism  for  the  special  purpose  of  accom- 
plishing a  singular  mode  of  leaping,  independently  of  any 
action  of  the  legs.  The  legs  of  this  insect  are  so  short,  that 
when  it  is  laid  on  its  back,  it  cannot  turn  itself,  beino-  unable 
to  reach  with  its  feet  the  plane  on  which  it  is  lying,  and 
procure  a  fulcrum  for  the  action  of  its  muscles.  It  is  appa- 
rently with  the  design  of  remedying  this  inconvenience, 
that  nature  has  bestowed  on  this  tribe  of  insects  the  faculty 
of  springing  into  the  air,  and  making  a  somerset,  so  as  to 
light  upon  the  feet;  an  effect  which  is  accomplished  by 
an  exceedingly  curious  mechanism.  The  prothorax  is  pro- 
longed beyond  the  length  it  usually  lias  in  other  coleop- 
tera,  and  it  is  articulated  with  the  mesothofax  on  the 
dorsal  side  by  two  lateral  tubercles,  which  form  a  hino-e 
joint,  limiting  its  motions  to  a  vertical  plane.  The  sternum, 
or  pectoral  portion  of  the  prothorax  is  also  extended  back- 
wards, and  terminates  in  an  elastic  spine,  which  is  received 
into  a  cavity  in  the  mesothorax,  and  which,  while  the  insect 
is  lying  on  its  back,  with  the  prothorax  bent  upon  the  meso- 
thorax, recoils  with  the  force  of  a  spring,  and  communicates 
to  the  body  an  impulse  which  carries  it  upwards  to  a  consi- 
derable height.  If  the  elater  should  fail  in  its  first  attempts 
to  recover  its  feet,  it  repeats  its  leaps  till  it  succeeds.  We 
find  no  example  of  a  similar  structure  in  any  other  part  of 
the  animal  kingdom. 

The  express  adaptation  of  structure  to  the  mode  of  life 
designed  for  each  species  of  insect  is  nowhere  more  strongly 
marked  than  in  those  which  are  intended  to  burrow  in  the 
earth:  and  of  these  the  Gryllo-ia/pa,  or  mole  cricket,  pre- 
sents a  remarkable  example.  A  minute  account  of  the  ana- 
tomy of  this  insect  has  been  given  by  Dr.  Kidd,"  from 
which  it  appears  that  being  destined,  like  the  mole,  to  live 
beneath  the  surface  of  the  earth,  and  to  excavate  for  itself  a 
passage  through  the  soil,  it  is  furnished  with  limbs  peculiar- 
ly calculated  for  burrowing,  with  a  skin  which,  being  co- 
vered with  a  fine  down,  effectually  prevents  the  adhesion  of 

•  Phil.  Trans,  for  1825,  p.  203. 
Vol.  I.  31 


242  THE  MECHANICAL  FUNCTIONS. 

the  moist  earth  through  which  it  moves,  and  with  a  form  of 
body  enabling  it  to  penetrate  with  least  resistance  the  oppo- 
sing medium.  By  being  endowed  with  the  power  of  moving 
as  easily  in  a  backward  as  in  a  forward  direction^  it  is  ena- 
bled quickly  to  retreat  in  the  narrow  channel  it  has  exca- 
vated: and  as  a  safeguard  in  these  retrograde  movements,  it  is 
provided  with  a  pair  of  posterior  appendages,  which  are  sup- 
plied with  large  nerves,  and  may  be  regarded  as  serving  the 
purpose  of  caudal  antennse. 

The  fore-legs,  (one  of  which  is  represented  in  Fig.  158"^) 
are  the  burrowing  implements,  and  they  are  admirably  cal- 
158*  .^^-^^^^^^  culated  for  their  peculiar  ofiice, 

%  ^  both  in  the  shape  and  in  the 
'-.^^  mode  of  articulation  of  their 
several  divisions,  which  bear  a 
considerable  analogy  to  the 
corresponding  member  of  the  mole.  Dr.  Kidd  observes, 
that,  compared  with  the  other  legs,  and  with  the  general  size 
of  the  animal,  they  are  as  if  the  brawny  hand  and  arm  of  a 
robust  dwarf  were  set  on  the  body  of  a  delicate  infant;  and 
the  indications  of  strength  which  their  structure  manifests, 
fully  answer  to  their  extraordinary  size.  For  a  more  par- 
ticular description  of  the  mechanism  of  this  instrument  I 
must  refer  the  reader  to  the  paper  above  quoted. 

§  9.  Flight  of  Insects. 

If  the  excellence  of  a  mechanic  art  be  measured  by  the 
difficulties  to  be  surmounted  in  the  attainment  of  its  object, 
none  surely  would  rank  higher  than  that  which  has  accom- 
plished the  flight  of  a  living  animal.  No  human  skill  has 
yet  contrived  the  construction  of  an  automaton,  capable,  by 
the  operation  of  an  internal  force,  of  sustaining  itself  in  the 
air  in  opposition  to  gravity,  for  even  a  few  minutes;  and 
far  less  of  performing  in  that  element  the  evolutions  which 
we  daily  witness  even  in  the  lowest  of  the  insect  tribes. 
To  the  ultimate  attainment  of  this  faculty  it  would  appear 
that  all  the  transformations  they  undergo  in  external  appear- 


PLIGHT  OF  INSECTS.  243 

ance,  and  all  the  developments  of  their  internal  mechanism, 
are  expressly  directed.     Wings  are  added  to  the  frame  only 
in  the  last  stage  of  its  completion;  after  it  has  disencum- 
bered itself  of  every  ponderous  material  that  could  be  spared, 
after  it  has  been  condensed  into  a  small  compass,  and  after 
it  has  been  perforated  in  all  directions  by  air-tubes,  giving 
lightness  and  buoyancy  to  every  part.     Curiously  folded  up 
in  the  pupa,  the  wings  there  attain  their  full  dimensions, 
ready  to  expand  whenever  the  bandages  that  surround  them 
are  removed.     No  sooner  is  the  insect  emancipated  form  its 
confinement,  than- these  organs,  which  are  composed  of  du- 
plicatures  of  a  dense  but  exceedingly  fine  membrane,  iden- 
tical in  its  composition  with  the  general  integuments,  begin 
to  separate  from  the  sides  of  the  body,  and  to  unfold  all 
their  parts.     Their  moisture  rapidly  evgiporates,  leaving  the 
delicate  film  dry  and  firm,  so  as  to  be  ready  for  immediate 
action.     The  fibres,  or  nervures,  as  they  are  called,  form  a 
delicate  net-work,  for  the  support  of  this  fine  membrane, 
like  the  frame  of  the  arms  of  a  windmill,  which  supports 
the  canvass  spread  over  them.     The  microscope  shows  that 
these  fibres  are  tubular,  and  contain  air;  a  structure  the  most 
effectual  for  conjoining  lightness  with  strength;  and  many 
entomologists  are  of  opinion  that  the  insect  has  the  power, 
during  the  act  of  flying,  of  directing  air  into  the  nervures, 
so  as  to  dilate  them  to  the  utmost,  and  render  them  quite 
tense  and  rigid. 

In  the  great  majority  of  insects,  the  wings  are  four  in 
number;  of  which  the  first  pair  are,  as  we  have  seen,  affixed 
to  the  mesothorax,  and  the  second  to  the  mctalhorax. 
These  two  segments  of  the  thorax,  composing  what  has  been 
termed  the  alitrunk,  constitute  the  most  solid  portion  of 
the  skeleton,  and  are  frequently  strengthened  hy  ridges, 
and  other  mechanical  contrivances  for  support.  The  su- 
perior extremities  of  these  supports,  which  have  been  com- 
pared to  the  clavicles,  or  furcular  bones  of  birds,  are  always 
curved  inwards.  This  part  of  the  trunk  requires  to  be  al- 
ternately dilated  and  contracted  during  flight;  and,  hence, 


244  THE  MECHANICAL  FUNCTIONS. 

the  several  pieces  of  which  its  dorsal  portion  is  composed, 
are  loosely  connected  together  by  ligaments.* 

The  shape  of  the  wings  is  more  or  less  triangular.  They 
are  moved  by  numerous  muscles,  which  occupy  a  large 
space  in  the  interior  of  the  trunk,  and  consist  of  various 
kinds  of  flexors,  extensors,  retractors,  levators,  and  depress- 
ors; the  whole  forming  a  very  complicated  assemblage  of 
moving  powers.  The  largest,  and  consequently  most  pow- 
erful of  these  muscles,  are  those  which  depress,  or  bring 
down  the  wings.  They  form  a  large  mass,  marked  a,  in 
Fig.  144.  All  these  muscles  exert  great  force  in  their  con- 
tractions, which  are  capable  of  being  renewed  in  very  rapid 
succession:  for,  indeed,  unless  they  had  this  power,  even  so 
lio-ht  a  body  as  that  of  an  insect  could  not  have  been  sus- 
tained for  a  moment  in  so  rare  a  medium  as  the  atmosphere, 
far  less  raised  to  any  height  by  its  resistance. 

The  simple  ascent  and  descent  of  the  wings  would  be  suf- 
ficient, without  any  other  movement  being  imparted  to  them, 
to  carry  forwards  the  body  of  the  insect  in  the  air.  The 
action  in  which  the  muscles  exert  the  greatest  force  is  in 
striking  the  air  during  the  descent  of  the  wing;  an  impulse 
in  the  opposite  direction  being  the  result  of  the  reaction  of 
the  air.  The  axis  of  motion  of  the  wings  is  a  line  inclined 
at  a  small  angle  to  the  axis  of  the  body,  and  directed,  from 
before,  backwards,  outwards, and  downwards;  and  they  move 
in  a  plane  which  is  not  vertical,  but  inclined  forwards.  The 
angle  which  the  plane  of  the  wing  forms  with  the  horizon 
varies  continually  in  the  different  positions  of  the  wing;  but 
the  general  resultant  of  all  these  successive  impulses  is  a 
force  directed  forwards  and  upwards;  the  first  part  of  this 
force  produces  the  horizontal  progression  of  the  insect,  while 
the  second  operates  in  counteracting  the  force  of  gravity; 
and  during  the  advance  of  the  insect,  either  maintains  it  at 
the  same  height,  or  enables  it  to  ascend. 

When  the  insect  wishes  to  turn,  or  to  pursue  an  oblique 

•  See  Chabrier's  "Essai  sur  le  Vol  des  Insectes,"  Memoires  du  Museum 
d'Histoire  Naturelle?  vi.  410,  vii.  297,  and  viii.  47  and  349.  See,  also,  Zoolo- 
gical Journal,  i.  391. 


FLIGHT  OF  INSECTS.  245 

course,  it  effects  its  purpose  very  easily  by  strikinir  the  air 
with  more  force  on  one  side  than  on  the  other;  or,  by  em- 
ploying certain  muscles  which  bend  the  body  to  one  side, 
it  shifts  the  situation  of  the  centre  of  gravity,  so  that  the  re- 
action of  the  air  on  the  wings  is  exerted  in  a  different  direc- 
tion to  what  it  was  before;  and  the  motion  of  the  body  is 
modified  accordingly. 

By  exerting  a  force  with  the  wings  just  sufficient  to  ba- 
lance that  of  gravity,  insects  can  poise  themselves  in  the  air, 
and  hover  for  a  length  of  time  over  the  same  spot,  without 
rising  or  falling,  advancing  or  retreating;  and  the  body  may, 
all  the  while,  be  kept  either  in  the  horizontal,  or  in  the  erect 
position.  In  the  latter  case,  the  motions  are  similar  to  those 
which  take  place  in  ordinary  flying,  only  they  are  more 
feebly  exerted,  since  all  that  is  required  is  to  sustain  the 
weight  of  the  body  without  urging  it  to  a  greater  speed. 
LibeUulde,  Sphinxes,  and  a  great  number  of  Diptera,  exhibit 
this  kind  of  action:  among  the  latter,  the  St7^aiiomys  is  most 
remarkable  for  its  power  of  remaining  long  in  the  same  fixed 
position. 

The  number,  form,  and  structure  of  the  wings  have  fur- 
nished entomologists  with  very  convenient  characters  for 
their  classification:  on  these  are  founded  the  orders  of  the 
Coleopiera,  Orthoptei^a,  Rhipiptera,  Hemiptera,  Neurop- 
tera,  Hymenoptera,  Diptera,  and  Lepidoptera.  To  enter 
into  any  detail  in  a  field  of  such  vast  extent  as  is  presented 
by  the  infinitely  diversified  mechanism  of  the  insect  crea- 
tion, would,  it  is  obvious,  far  exceed  the  proper  limits  of 
this  treatise.  I  must,  therefore,  confine  myself  to  a  few  lead- 
ing points  in  their  structure  and  modes  of  progression. 

In  the  Coleopiera,  an  order  which  comprehends  by  far  the 
largest  number  of  genera  of  insects,  the  lower  pair  of  wino-s 
(w,  Fig.  150,  p.  228)  are  light  and  membranous,  and  of  a 
texture  exceedingly  fine  and  delicate.  They  are  of  great 
extent,  compared  with  the  size  of  the  body,  when  fully  ex- 
panded: and  are  curiously  folded  when  not  in  use.  For  the 
protection  of  these  delicate  organs,  the  parts  which  cor- 
respond to  the  upper  pair  of  wings  of  other  insects,  are  here 


246 


THE  MECHANICAL  FUNCTIONS. 


converted  into  thick  opaque,  and  hard  plates,  (e,)  adapted  to 
cover  the  folded  membranous  wings  when  the  insect  is  not 
flying,  and  thus  securing  them  from  injurious  impressions, 
to  which  they  might  otherwise  be  exposed  from  heat,  miois- 
ture,  or  the  contact  of  external  bodies.  These  wing  cases, 
or  elytra^  as  they  are  termed,  are  never  themselves  em- 
ployed as  wings,  but  remain  raised  and  m.otionless  during 
the  flight  of  the  insect.  They  probably,  however,  contri- 
bute to  direct  the  course  of  flight,  by  variously  modifying 
the  resistance  of  the  air.* 

In  the  Orthoptera,  (Fig.  159,)  the  coverings  of  the  wings, 
or  tegmina,  instead  of  being  of  a  horny  texture,  are  soft  and 
flexible,  or  semi-membranous.  The  wings  themselves,  being 
broader  than  their  coverings,  are,  when  not  in  use,  folded 
longitudinally,  like  a  fan. 

In  the  new  Order  of  Rhipiptera  of  Latreille,t  which  in- 
cludes only  two  genera,  the  tegmina  are  anomalous  both  in 


160 


159 


162 


their  situation  and  shape;  being  fixed  at  the  base  of  the  an- 
terior legs,  very  long  and  narrow,  and  apparently  incapable 
of  protecting  the  wings.     The  wings  themselves  are  of  am- 


*  The  Elytra  of  insects  have  been  regarded,  by  Oken,  as  corresponding  to 
the  bivalve  shells  of  the  Mollusca,  a  notion  which  seems  to  be  founded  upon 
a  fanciful  and  strained  analogy. 

f  The  Strepsipiera  of  Kirby.  See  Transactions  of  the  Linnjean  Society, 
XI.  86. 


FLIGHT  OF  INSECTS.  247 

pie  extent,  forming,  when  expanded,  a  quadrant  of  a  circle, 
with  five  or  six  nervurcs  radiating  from  their  base,  and 
folded  longitudinally. 

In  the  Hetniptera,  the  tegmina,  or  as  they  are  here 
called,  the  hemi-elytra^  are  coriaceous  towards  their  base, 
but  membraneous  towards  their  extremity,  and  the  true 
wings  are  folded  transversely,  so  as  to  cross  one  another. 
These  hemi-elytra  are  employed  to  strike  the  air  in  flight, 
and  their  movements  accompany  those  of  the  wings. 

Insects  having  four  thin  membranous  and  transparent 
wings  are  arranged  under  two  orders;  namely,  the  Neurop- 
tera  (Fig.  160,)  in  which  the  lesser  nervures  form  an  inter- 
lacement of  fibres,  crossing  one  another  nearly  at  right 
angles,  like  net-work,  or  lace:  and  the  Hymenoptcra  (Fig. 
161,)  in  which  they  are  disposed  like  the  ramifications  of 
arteries  or  veins,  diverging  at  acute  angles  from  the  main 
trunks.  The  insects  belonging  to  these  two  orders  enjoy 
extensive  powers  of  flight.  Libellulse,  and  JEschnse^  which 
are  included  in  the  first  of  these  orders,  never  close  their 
wings,  but,  when  they  are  not  flying,  keep  them  constantly 
expanded,  and  ready  for  instant  action.  They  fly  with  the 
greatest  ease  in  all  directions,  sideways,  or  backwards,  as 
well  as  forwards,  and  can  instantly  change  their  course  with- 
out being  obliged  to  turn  their  bodies.  Hence  they  possess 
great  advantages  both  in  chasing  other  insects,  and  in  evading 
the  pursuit  of  birds.  Bees^  which  are  hymenopterous  in- 
sects, have  often  been  observed  to  fly  to  great  distances 
from  their  hive  in  search  of  food.  The  humble  bee  adopts 
a  very  peculiar  mode  of  flight,  describing,  in  its  aerial  course, 
segments  of  circles,  alternately  to  the  right  and  to  the  left. 
The  velocity  with  which  these  insects  move  through  the 
air,  in  general,  much  exceeds  that  of  a  bird,  if  estimated  with 
reference  to  the  comparative  size  of  these  animals.* 

*  I  have  been  favoured  by  Mr.  George  Newport  with  the  following  ac- 
count of  the  structure  of  the  sting  of  the  Wild  Bee,  {Anthophora  rctusOy 
Kirby)  which  he  has  lately  carefully  examined,  and  from  whose  drawings 
of  the  dissected  parts  the  annexed  figures  (163)  have  been  engraved,  "  The 
sting  of  the  bee,  a,  is  formed  of  two  portions  placed  laterally  together,  but 


248 


THE  MECHANICAL  FUNCTIONS. 


Although  the  greater  number  of  insects  have  four  wings, 
there  are  many,  such  as  the  common  house  fly,  and  the  gnat, 
which  have  only  two.  These  compose  the  order  Diptera 
(Fig.  162.)  In  these  insects  we  meet  with  two  organs,  con- 
sistino"  of  .cylindrical  filaments,  terminated  in  a  clubbed  ex- 
tremity; one  arising  from  each  side  of  the  thorax  (as  seen  in 
the  above  figure,)  in  the  situation  in  which  the  second  pair 
of  wings  originate  in  those  insects  that  have  four  wings. 

capable  of  being-  separated.  The  point,  v,  is  directed  a  little  upwards,  and 
is  a  little  curved:  the  barbs,  seen  still  more  highly  magnified  at  a,  are  about 
six  in  number,  and  are  placed  on  the  under  surface,  and  their  points  directed 
backwards.     At  the  base  of  the  sting-,  e,  there  is  a  semicircular  dilatation 

apparently  intended  to  prevent  the  in- 
strument from  being  thrust  too  far  out 
of  the  sheath  (seen  separately  at  y,)  in 
which  it  moves:  it  has  also  a  long  ten- 
don to  v/liich  the  muscles  are  attached. 
It  is  between  these  plates,  when  ap- 
proximated, that  the  poison  flows  from 
the  orifice  of  the  somewhat  dilated  ex- 
tremity of  the  poison  duct,  D,  which 
comes  from  the  anterior  part  of  the 
poison  bag,  b.     This  bag  is  of  an  oval 
shape,  and  is  not  the  organ  which  se- 
cretes the  poison,  but  merely  a  recep- 
tacle for  containing  it:  for  it  is  conveyed 
into  this  bladder  by  means  of  a  long  con- 
voluted vessel,  c,  which  receives  it  from 
the  secreting  organs,  s.     These  organs 
consist  of  two  somewhat  dilated  vessels 
resembling  caeca,  but  which  have  each  a 
slender  secretory  vessel  extending  from 
them.     The  sting  moves  in  a  tubular 
sheath,  v;  which  is  open  at  its  base,  and 
along  its  upper  surface,  as  far  as  the 
part  where  the  sting  is  prevented  from 
being  thrust  out  any  farther.    The  mus- 
cles which  move  the  sheath  are  distinct  from  those  of  the  sting,  and  are  at- 
tached  to  an  elongated  and  curved  part  on  each  side  of  its  base,  and  to  an 
arched  and  moveable  part  which  is  apparently  articulated  with  it.     Swam- 
merdam  has  delineated  these  parts  as  caeca  in  his  dissection  of  the  common 
hive  bee,  but  has  not  noticed  the  secretory  vessels.     The  sting  of  the  hive 
bee  resembles  that  of  the  Anthophora  retusa.'' 


PLIGHT  or  INSECTS.  249 

They  are  named  the  hdUeres,  or  poisers,  from  their  sup- 
^Dosed  use  in  balancing  the  body,  or  adjusting  with  exactness 
the  centre  of  gravity  when  the  insect  is  flying.  Whatever 
may  be  their  real  utility,  they  may  still  be  regarded  as  rudi- 
ments of  a  second  pair  of  wings;  and  they  afford,  therefore, 
when  thus  viewed,  a  striking  instance  of  the  operation  of 
the  tendency  which  prevails  universally  in  the  animal  king- 
dom, and  modifies  the  structure  of  each  individual  part  so  as 
to  preserve  its  conformity  to  one  general  type. 

The  innumerable  tribes  of  butterflies,  sphinxes,  and  moths, 
are  all  comprehended  in  the  order  Lcjndoptcra,  and  are  dis- 
tinguished by  having  wings  covered  with  minute  plumes  or 
scales.  These  scales  are  attached  so  slightly  to  the  membrane 
of  the  wing  as  to  come  off  when  touched  with  the  fingers,  to 
which  they  adhere  like  fine  dust.  When  examined  with 
the  microscope,  their  construction  and  arrangement  a[)pear 
to  be  exceedingly  beautiful,  being  marked  with  parallel  and 
equidistant  striae,  often  crossed  by  still  finer  lines,  the  dis- 
tinct visibility  of  which  in  many  kinds  of  scales,  as  those  of 
Pontia  brassica,  or  cabbage  butterfly,  and  the  Morp/io 
Menelaus  of  America,  constitutes  a  good  criterion  of  the  ex- 
cellence of  the  instrument.  The  beautiful  colours  which 
these  scales  possess  may  perhaps  generally  be  owing  to  the 
presence  of  some  colouring  material:  but  the  more  delicate 
hues  are  probably  the  result  of  the  optical  effect  of  the  striai 
on  the  surface;  and  in  some  cases  they  result  from  the  thin- 
ness of  the  transparent  plate  of  which  they  consist;  for  I 
have  observed  in  several  detached  scales  that  the  colours  they 
exhibit  by  transmitted  light  are  the  complementary  colours 
to  those  which  they  display  when  seen  by  reflected  liglit- 

The  forms  of  these  scales  arc  exceedingly  diversified,  not 
only  in  different  species,  but  also  in  different  parts  of  the 
wings  and  body  of  the  same  insect;  for  the  surface  of  the 
body,  generally,  as  well  as  the  limbs,  and  even  in  some  spe- 
cies the  antennae  are  more  or  less  covered  with  these  scales.* 

•  In  the  posthumous  work  of  Lyonet,  which  lias  lately  appeared,  nearly  the 
whole  of  six  quarto  plates  are  crowded  with  the  delineations  of  tlic  dillcrent 
forms  of  the  scales  found  in  the  Bomhyx  Cossus. 

Vol.  I.  32 


250 


THE  MECHANICAL  FUNCTIONS. 


Fig.  164  exhibits  some  of  the  more  usual  shapes  as  they  ap- 
pear when  viewed  with  high  magnifying  powers. 

Each  scale  is  inserted  into  the  membrane  of  the  wing  by 
a  short  pedicle,  or  root,  and  overlaps  the  adjoining  scales: 
and  the  wliole  are  disposed  in  rows  with  more  or  less  regu- 


"^  V  ^ 


larlty;  one  row  covering  the  next,  like  tiles  on  the  roof  of  a 
house.*  This  imbricated  arrangement,  together  with  the 
marks  that  are  left  on  the  membrane  of  the  wing  where  the 
scales  have  been  rubbed  off,  are  shown  in  Fig.  165,  which 
is  a  faithful  delineation  of  the  appearance  of  the  wing  of  the 
Hesperia  Sloanus,  seen  through  a  powerful  microscope. 
The  membrane  of  the  wing  itself,  when  stripped  of  its 
scales,  is  as  perfectly  transparent  as  that  of  the  bee,  and  is, 
in  like  manner,  supported  by  diverging  nervures.  Many 
butterflies  exhibit  in  some  parts  of  the  wing  smooth  pearly 
spots,  called  by  entomologists,  ocelli,  or  eyes,  which  arise 
from  those  parts  being  naturally  destitute  of  scales.  The 
number  of  these  scales  necessary  to  cover  the  surface  of  the 
wings  must,  from  their  minuteness,  be  exceedingly  great. 
The  moth  of  the  silk  worm   {Bornbyx  mori,  Fig.    148,) 

*  The  scales  on  the  wing  of  the  Leplsma  are  of  two  kinds;  one  set  being 
arranged  in  rows,  as  usual,  and  the  otliers,  which  are  of  a  different  shape, 
being  inserted  between  and  over  the  former,  so  as  to  fasten  each  firmly  in  its 
place. 


FLIGHT  OF  INSECTS.  05^ 

which  has  hut  a  small  wine;,  contains,  according  to  Lewen- 
hoeck,  more  than  two  hundred  thousand  of  these  scales  in 
each  wing. 

These  scales  douhtless  contrihute  to  the  protection  of  the 
wing;  but  they  at  the  same  time  add  considerably  to  their 
weight,  and  impede  the  velocity  of  their  action.  This  in- 
convenience appears  to  have  been  in  a  great  measure  com- 
pensated by  the  greater  size  of  the  wings,  and  by  the  extent 
of  the  surface  with  which  they  strike  the  air.  Still,  how- 
ever, it  is  sufficiently  obvious  that  insects  of  this  order  fly 
with  less  rapidity  and  steadiness  than  most  others.  But  this 
unsteadiness,  again,  is  turned  to  good  account;  for  the  but- 
terfly, by  its  irregular  and  apparently  capricious  movements, 
alternately  dipping  and  rising  in  the  air,  so  as  to  describe  a 
series  of  zigzag  lines,  more  easily  eludes  capture  when  pur- 
sued, not  only  by  naturalists,  but  also  by  birds  that  are  ea- 
gerly seeking  to  secure  them.  It  is  astonishing  to  what  a 
distance  the  silk  worm  motiis  will  fly:  some  have  been 
known  to  travel  more  than  a  hundred  miles  in  a  short  time. 
The  PapUio  Iris  often  rises  to  so  great  a  heiglit  in  the  air 
as  to  be  quite  invisible. 

A  mechanical  contrivance  is  adopted  in  many  of  the  Le- 
pidoptera  for  keeping  their  wings  steady  during  flight,  con- 
sisting of  a  hook  covered  with  hair  and  scales,  attached  to 
the  under  side  of  the  upper  wings  near  their  ijase,  and  con- 
nected also  by  means  of  bristles  to  the  base  of  the  lower 
wing:  by  this  attachment. all  the  wings  are  locked  together 
and  brought  into  action  at  the  same  time.  Insects  of  the 
Sphinx  tribe  are  also  provided  with  a  kind  of  rudder  formed 
by  the  expansion  of  the  tail,  enabling  them  to  steer  their 
course  with  more  certainty.  The  Lcpidojitera  in  general 
fly  with  the  body  nearly  upright,  contrary  to  the  habits  of 
most  other  winged  insects,  whose  bodies,  while  flying,  are 
nearly  in  a  horizontal  position. 

The  feats  of  agility  and  strength  exhibited  by  insects  have 
often  been  the  theme  of  admiration  with  writers  on  natural 
history;  and  have  been  considered  asaflbrding  incontrovert- 
ible proofs  of  the  enormous  power  with  which  their  muscles 


252  THE  MECHANICAL  FUNCTIONS. 

must  be  endowed.  We  have  already  had  occasion  to  notice 
a  remarkable  instance  of  the  force  and  permanence  of  mus- 
cular contraction  in  tliose  caterpillars  which  frequently  re- 
main for  hours  together  in  a  fixed  attitude,  with  their  bodies 
extended  from  a  twig,  to  which  they  cling  with  their  hind 
feet  alone.*  Ants  will  carry  loads  which  are  forty  or  fifty 
times  heavier  than  their  own  bodies:  and  the  distance  to 
which  many  species,  such  as  the  Elater,  the  Locust,  the 
Lepisma,  and  above  all  the  Pidex,  are  capable  of  leaping, 
compared  with  the  size  of  the  insects  themselves,  appear 
still  more  astonishing.  Linnaeus  has  computed  that  the 
Melolontha,  or  chaffer,  is,  in  proportion  to  its  bulk,  more 
than  six  times  stronger  than  the  horse:  and  has  asserted  that 
if  the  same  proportional  strength  as  is  possessed  by  the  Lu- 
canus,  or  stag-beetle,  had  been  given  to  the  elephant,  that 
animal  would  have  been  capable  of  tearing  up  by  the  roots 
the  largest  trees,  and  of  hurling  huge  rocks  against  his  as- 
sailants, like  the  giants  of  ancient  mythology. 

But  while  we  must  admit  that  all  these  facts  indicate  a  re- 
markable degree  of  energy  in  the  contractile  power  of  the 
miuscular  fibres  of  insects,  we  should  at  the  same  time  re- 
collect that  the  diminutive  size  of  the  beings  w^hich  display 
those  powers  is  itself  the  source  of  a  mechanical  advantage 
not  possessed  by  larger  animals.  The  efficacy  of  all  mechani- 
cal arrangements  must  ultimately  depend  on  a  due  propor- 
tion between  the  moving  and  the  resisting  forces:  hence 
mechanism  of  every  kind  must  be  adjusted  with  reference 
not  merely  to  the  relative,  but  to  the  absolute  dimensions 
of  the  structures  themselves.  This  will  be  evident  when 
we  consider  that  the  forces  which  are  called  into  action  are 
resisted  by  the  cohesion  of  the  particles  composing  the  solid 
parts  of  the  machine:  and  this  cohesion  being  not  a  variable, 
but  a  constant  and  definite  force,  must  necessarily  limit  the 
dimensions  of  every  mechanical  structure,  whether  intend- 
ed for  stability  or  for  action.  An  edifice  raised  beyond  a 
certain  magnitude,  will  not  support  itself,  because  the  weight 

*  See  Fig-  148*,  p.  224. 


FLIGHT  OF  INSECTS.  253 

of  the  materials  increases  more  rapidly  than  the  strength. 
How  often  has  it  been  found  that  a  machine  which  works 
admirably  in  a  small  model,  will  totally  fail  in  its  perform- 
ance when  constructed  on  a  larger  scale?  x\ny  lever,  of 
whatever  form,  may  be  increased  in  its  dimensions  until  the 
force  of  gravity  becomes  superior  to  the  cohesion  of  its  own 
particles:  and  consequently  any  structure,  like  a  vegetable 
or  animal  body,  composed  of  a  combination  of  levers,  would, 
if  its  size  were  to  exceed  a  certain  limit,  fall  to  pieces  mere- 
ly by  its  own  weight.  This  can  be  prevented  either  by  em- 
ploying materials  of  greater  cohesive  strength,  or  by  in- 
creasing, at  the  points  where  the  strains  are  greatest,  the  thick- 
ness of  the  parts  compared  with  their  length:  but  the  choice 
of  materials  is  necessarily  restricted  within  narrow  limits, 
and  the  latter  expedient  would  entirely  alter  the  relative 
proportions  of  the  parts,  and  would  require  a  complete 
change  in  the  plan  of  their  construction.  In  passing  from 
the  smaller  to  the  larger  animals,  we  find,  accordingly,  that 
new  models  are  adopted,  a  new  order  of  architecture  intro- 
duced, and  new  laws  of  development  observed.  "We  have 
next,  then,  to  direct  our  attention  to  the  procedure  of  na- 
ture in  the  execution  of  this  more  enlarged  and  comprehen- 
sive scheme  of  animal  organization. 


(     254     ) 


CHAPTER  VL 

vertebrata, 
?»  1.    Vertebrated  Animals  in  general 


§ 


If  it  be  pleasing  to  trace  the  footsteps  of  nature  in  con- 
structions so  infinitely  varied  as  those  of  the  lower  animals, 
and  to  follow  the  gradations  of  ascent  from  the  zoophyte  to 
the  winged  insect,  which  exhibits  the  greatest  perfection  com- 
patible with  the  restricted  dimensions  of  that  class  of  beings, 
still  more  interesting  must  be  the  study  of  those  more  ela- 
borate eflbrts  of  creative  power,  which  are  displayed,  on  a 
wider  field,  in  the  higher  orders  of  the  animal  kingdom.  In 
the  various  tribes  of  beings  which  are  now  to  come  before 
us,  we  find  nature  proceeding  to  display  more  refined  deve- 
lopments in  her  system  of  organization,  resorting  to  new 
models  of  structure  on  a  scale  of  greater  magnitude  than  be- 
fore, devisino;  new  plans  of  economy,  calculated  for  more  ex- 
tended periods  of  duration,  and  adopting  new  arrangements 
of  organs,  fitted  for  the  exercise  of  a  higher  order  of  facul- 
ties.    The  result  of  these  more  elaborate  constructions  is 
seen  in  the  vast  series  of  Vertebrated  Animals,  wdiich  com- 
prises a  well-marked  division  of  Zoology,  comprehending 
all  the  larger  species  that  exist  on  the  globe,  in  whatever 
climate  or  element  they  may  be  found:  and  including  man 
himself,  placed,  as  he  unquestionably  is,  at  the  summit  of 
the  scale; — the  undisputed  Lord  of  the  Creation. 

A  remarkable  affinity  of  structure  prevails  throughout  the 
whole  of  this  extensive  assemblage  of  beings.  Whatever 
may  be  tlie  size  or  external  form  of  these  animals,  whatever 
the  activity  or  sluggishness  of  their  movements,  whether 
they  be  inhabitants  of  the  land,  the  waters,  or  the  air,  a 
striking  similitude  may  be  traced,  both  in  the  disposition  of 
their  vital  organs,  and  in  the  construction  of  the  solid  frame- 


VERTEBRATED  ANIMALS.  255 

work,  or  skeleton,  which  sustains  and  protects  their  fabric. 
The  quadruped,  the  bird,  tlie  tortoise,  the  serpent,  and  the 
fish,  however  they  may  differ  in  subordinate  details  of  or- 
ganization, are  yet  constructed  upon  one  uniform  principle, 
and  appear  like  varied  copies  from  the  same  original  model. 
In  no  instance  do  they  present  structures  which  are  altoge- 
ther isolated,  or  can  be  regarded  as  the  results  of  separate 
and  independent  formations. 

In  proceeding  from  the  contemplation  of  the  structures  of 
articulated  to  those  of  vertebrated  animals,  we  appear  to  pass, 
by  a  rapid  excursive  flight,  from  one  great  continent  to  ano- 
ther, separated  by  an  immense  gulf,  containing  no  interme- 
diate islands  from  which  we  might  gather  indications  of  these 
tracts  of  land  having  been  originally   connected.     At  the 
very  first  sight,  indeed,  the  general  fabrics  of  these  two  de- 
scriptions of  animals  appear  to  have  been  constructed  upon 
opposite  principles;  for,  in  the  one,  as  we  have  already  seen, 
the  softer  parts  are  internal,  and  are  enclosed  in  a  solid  crust, 
or  shell,  or  horny  covering,  answering,  at  once,  the  purposes 
of  protection  and  mechanical  support,  and  furnishing  exten- 
sive surfaces  for  the  attachment  of  the  orjrans  of  motion. 
But,  in  the  Vertebrata,  the  solid  frame-work  which  serves 
these  purposes,  occupies,  for  the  most  part,  an  internal  situa- 
tion, constituting  a  true-jointed  skeleton,  which  is  surround- 
ed by  the  softer  organs,  and  to  which  the  muscles,  destined 
to  move  their  several  parts,  are  attached.     The  office  of  ex- 
ternal defence  is  intrusted  solely  to  the  integuments,  and 
their  different  appendages.     Such  is  the  general  character  of 
the   arrangements   which   nature    has    here   ado])ted;   from 
which,  however,  she  has  occasionally  deviated  with  respect 
to  some  important  organs  of  extremely  delicate  texture,  and 
which  require  to  be  shielded  from  the  slightest  pressure. 
This  occurs  with  regard  to  the  brain,  and  the  spinal  marrow, 
which,  we  shall  presently  find,  are  specially  guarded  by  a 
bony  structure,  enclosing  them  on  every  side,  and  forming 
an  impenetrable  case  for  their  protection.     The  solid  mass 
of  bone,  thus  provided  to  defend  the  brain,  gives  also   the 
opportunity  of  lodging  safely  the  delicate  apparatus  subscr- 


256  THE  MECHANICAL  FUNCTIONS. 

vient  to  the  finer  senses,  namely,  those  of  siglit,  of  hearing, 
and  of  smell.  The  security  which  these  organs  derive  from 
this  protection  allows  of  their  being  carried  to  a  higher  de- 
gree of  improvement  than  could  be  attained  in  the  lower  or- 
ders. 

There  is  also  another  advantage,  of  considerable  moment, 
which  results  from  the  internal  situation  of  the  skeleton, 
namely,  that  it  admits  of  an  indefinite  extension  by  growth, 
without  interfering  with  the  corresponding  enlargement  of 
the  softer  organs;  for  we  have  seen  that  in  all  the  instances 
in  which  this  arrangement  is  reversed,  that  is,  whenever  the 
enclosing  surfaces  become  solid,  and  can  no  longer  yield  to 
the  dilatation  of  the  contained  organs,  no  alternative  remains 
but  that  of  breaking  up  the  exterior  case,  and  vv-holly  cast- 
ing it  off*,  to  make  room  for  the  farther  growth  of  the  ani- 
mal; after  which  operation,  it  has  to  be  replaced  by  another 
coverincr  of  larger  dimensions.  This  operation  is  generally 
required  to  be  performed  a  great  number  of  times,  before 
the  animal  can  acquire  the  size  it  is  destined  to  attain. 
Hence  the  perpetual  moultings  of  the  caterpillar;  hence  the 
repeated  castings  of  the  shells  of  the  Crustacea;  and  hence 
also  the  successive  metamorphoses  of  the  insect.  Nothing 
of  this  kind  takes  place  among  the  Vertebrata;  where  all  the 
organs  are  developed  in  regular  and  harmonious  succession, 
without  the  slighest  mutual  interference,  and  without  those 
vicissitudes  of  action,  and  of  torpidity,  which  we  witness  in 
the  chequered  existence  of  the  insect. 

§  2.  Structure  and  Comjiositioii  of  the  Osseous  Fabric. 

The  process  employed  for  the  formation  and  extension  of 
the  solid  frame-work  of  the  Vertebrata  diff'ers  totally  from 
that  which  we  have  seen  exemplified  in  the  growth  of  shells, 
or  of  the  hard  coverings  of  insects  and  of  crustaceous  ani- 
mals. These  latter  structures,  and  the  modes  adopted  for  their 
increase,  are  suited  only  to  animals  in  which  the  functions 
of  the  economy  have  not  reached  that  perfection  to  w^hich 
they  are  carried  in  the  higher  classes.     In  the  more  elabo- 


CHEMICAL  COMPOSITION  OF   BONE.  257 

rate  system  of  the  vcrtcbrata,  the  skeleton  is  composed  of 
true  bones;  that  is,  of  solid  pieces,  which,  althouirh  they  arc 
dense  calcareous  structures,  yet  continue  organized  during 
the  whole  period  of  development,  and  form  as  much  a  part 
of  the  living  system  as  any  other  organ  of  the  body.  We 
have  formerly  seen  that  the  membrane  in  which  the  calca- 
reous matter  of  the  shell  is  deposited,  should  properly  be 
classed  among  the  integuments;  being  analogous  to  them  not 
only  in  being  situated  externally,  but  also  in  their  structure 
and  in  their  function.  It  is  not  so  with  bone,  which  is 
essentially  an  internal  structure.* 

In  their  chemical  composition, likewise,bones  are  striking- 
ly contrasted  with  the  calcareous  products  of  the  INIolIusca: 
for  in  the  former,  the  earthy  portion  consists  almost  wholly 
of  phosphate  of  lime:  a  material  which  appears  to  have  been 
selected  for  this  purpose  from  its  forming  much  harder  com- 
pounds with  animal  membrane  than  the  carbonate.  Where- 
ver great  strength  and  rigidity  are  required,  this  is  the  ma- 
terial depended  on  for  imparting  these  qualities;  and  it  has, 
accordingly,  been  employed  for  the  osseous  structures,  which 
are  among  the  most  elaborate  results  of  organization.  The 
densest  and  hardest  of  these  structures  are  those  in  which 

*  De  Blainville  rcg-ards  the  hard  covering's  of  insects,  tog-ether  with  the 
shells  of  the  Crustacea,  as  structures  derived  aUog-cther  from  the  intcg-uments, 
and  as  perfectly  anaiog-ous,  in  this  respect,  to  the  scales,  hoofs  or  other  horny 
productions  of  the  skin  in  vertebrated  animals.     Geoffrey  St.  Hilaire  con- 
tends, on  the  contrary,  that  the  former  constitute  the  true  skeleton  of  the 
lower  classes,  and  that  a  perfect  analogy  may  be  traced  between  the  ring-s, 
which  arc  the  essential  constituents  of  the  frame-work  of  annulose  animals, 
and  the  vertebrae,  which  enclose  the  spinal  cord  of  the  higher  classes.     Pro- 
fessor Carus  appears  in  his  system  of  org-anic  formations,  to  have  kept  in  view 
both  these  analogies;  g-iving-  to  the  former  class  of  structures  the  denomina- 
tion of  Uermo-skeleioJi,  and  to  the  latter  that  of  Netiro-skeklon  (See  his  Ta- 
bulx  Anatomiam  Comparativam  illustrantes,  edited  by  Thienemann.)    Ana- 
log'ies  have  also  been  imag-ined  to  exist  between  the  external  and  internal 
situations  of  the  woody  fibres  of  plants  belong-ing  respectively  to  the  endoge- 
nous and  exogenous  classes,  and  that  of  the  corres[)onding  relative  situations 
of  the  skeletons  of  invertebrated  and  vertebrated  animals.     Sec  a  Memoir  by 
Dumortier,  in  the  Nova  Acta  Physico-Medica  Acad.  Ciesar.  Leopold.  Caro- 
lina Natur.  Curios.  XVL,  219.) 

Vol.  I.  33 


258  THE  MECHANICAL  FUNCTIONS. 

the  proportion  of  phosphate  of  lime  is  the  greatest,  when 
compared  with  that  of  the  animal  substance  which  cements 
them  together;  the  force  of  mutual  cohesion  among  its  own  . 
particles  being  much  greater  than  that  imparted  by  the  ce- 
menting ingredient.     The  internal  bony  portions  of  the  ear, 
where,  in  order  perfectly  to  transmit  the  sonorous  vibra- 
tions, the  greatest  solidity  is  required,  are  the  densest  parts 
of  the  skeleton;  and  phosphate  of  lime  enters  most  largely 
into  the  composition  of  these  bones.     The  tympanic  portions 
of  the  temporal  bone  of  the  whale  and  the  cachalot,  where 
the  great  size  of  the  organ  gives  us  advantages  in  examining 
them,  are  as  dense  and   as  hard  as  marble.     The  bony  por- 
tions of  the  teeth,  likewise,  afford  instances  of  very  hard  cal- 
careous formations;  but  the  enamel,  which  consists  almost 
wholly  of  phosphate  of  lime,  is  harder  still,  and  resembles 
the  siliceous  stones,  being,  like  flint,  capable  of  striking  fire 
wdth  steel.     It  is  scarcely  necessary  to  point  out  the  obvious 
intentions  which  are  fulfilled  by  this  peculiarity  of  structure, 
conferring  extraordinary  hardness  on  a  part,  of  which  the 
appropriate  office  is  that  of  breaking  down  hard  bodies  sub- 
jected to  their  mechanical  action.     But  this  extreme  degree 
of  crystalline  hardness  v/ould  be  ill-suited  to  other  parts  of 
the  frame.     In  ordinary  bones,  absolute  rigidity  is  not  the 
quality  which  is  alone  wanted;  for,  in  general,  the  hardest 
bodies  are  also  the  most   fragile.     An  excess  of  rigidity, 
therefore,  would  have  been  attended  with  brittleness,  and 
been  productive  of  the  worst  consequences  to  parts  exposed 
to  sudden  and  violent  concussions.     It  is  in  order  to  guard 
against  this  evil  that  an  elastic  animal  matter  is  employed  as 
the  basis  of  the  structure,  acting  as  a  strong  cement  inter- 
posed between  the  calcareous  particles. 

This  composition  of  bone  is  rendered  evident  by  subject- 
ing it  to  certain  chemical  processes.  On  exposure  to  heat, 
we  find  it  first  becoming  black,  from  the  development  of 
the  charcoal  attendant  upon  the  destruction  of  the  animal 
membrane.  The  oil  contained  in  the  cavities  exudes,  and, 
taking  fire,  is  soon  totally  consumed.  The  bone  then  reco- 
vers its  whiteness,  and  undergoes  no  farther  change  by  the 


CHEMICAL  COMPOSITION  OF  BONE.  259 

action  of  llic  fire.  If  it  be  now  examined,  it  will  be  found 
to  have  lost  nearly  half  its  original  weight,  and  to  have  be- 
come exceedingly  brittle;  this,  as  already  mentioned,  being 
the  natural  ])roperty  of  phosphate  of  lime,  when  deprived  of 
its  animal  cement.  We  may  perceive  on  the  surface  of  a 
bone  so  treated,  a  number  of  minute  crevices,  showinc:  where 
this  animal  substance  had  been  situated,  in  its  original  state. 
On  breaking  the  bone  across,  we  may  also  discover  the  size 
and  shape  of  the  cavities  which  contained  the  marrow,  or 
oily  fluid  above  mentioned. 

It  is  easy  to  reverse  this  process  by  steeping  the  bone  in 
an  acid  sufficiently  diluted  to  prevent  its  injuring  the  animal 
membrane,  but  yet  sufficiently  powerful  to  dissolve  the 
phosphate  and  carbonate  of  lime.  Diluted  nitric  or  muria- 
tic acids  may  be  used  for  this  purpose,  and  will,  in  this  way, 
gradually  separate  the  earthy  particles  from  the  membranous 
portion  of  the  bone.  During  the  action  of  the  acid  a  few 
bubbles  of  carbonic  acid  gas  make  their  appearance,  indi- 
cating the  presence  of  a  small  quantity  of  carbonate  of  lime, 
which  always  exists  in  bones,  intermixed  with  the  phos- 
phate. The  phosphate  may  be  recovered  from  its  solution 
in  the  acid  by  precipitation  with  a  pure  alkali,  such  as  a  so- 
lution of  ammonia.  This  precipitate  is  readil}'  dissolved, 
without  effiirvescence,  by  nitric,  muriatic,  or  acetic  acids. 
A  small  quantity  of  sulphuric  acid  may  also  be  detected  in 
the  fluid  by  the  addition  of  nitrate  of  barytes.  Iron,  in 
small  quantity,  is  also  found  in  the  composition  of  human 
bones. 

The  substance  which  remains,  after  the  earth  has  been 
thus  abstracted,  retains  the  exact  figure  and  dimensions  of 
the  original  bone,  but  has  lost  all  its  other  mechanical  pro- 
perties. It  is  soft,  flexible,  and  elastic;  resembling  in  every 
respect  the  muscular  or  fibrous  structures,  and  being,  like 
them,  resolvable  into  gelatin  and  albumen  by  long  boiling 
in  water.  This  substance  has  sometimes,  but  erroneously, 
been  considered  as  identical  with  cartilage;  for  it  has  nei- 
ther the  whiteness,  nor  the  elasticity,  nor  the  texture  of  carti- 
lage, nor  is  it  at  all  similar  to  that  substance  in  its  chemical 


260  THE  MECHANICAL    FUNCTIONS. 

composition:  for  while  cartilage  isjormed  almost  wholly  of 
albumen,  the  animal  basis  of  bone  is  almost  entirely  resol- 
vable into  gelatin. 

Thus  may  a  bone  be  analyzed  into  its  two  constituent 
parts:  by  the  process  first  described  we  obtain  its  earth  de- 
prived of  its  animal  constituent;  by  the  second,  we  obtain 
its  membranous  basis  free  from  earth.  The  first  of  these 
gives  it  hardness;  the  second,  tenacity:  and  thus,  by  the  in- 
timate combination  of  these  elements,  two  qualities,  which, 
in  masses  of  homogeneous  and  unorganized  matter,  are 
scarcely  compatible  with  one  another,  are  skilfully  united. 

The  mechanical  structure  of  bone  is  no  less  worthy  of  ad- 
miration, as  evincing  the  skill  with  which  every  part  is 
adapted  to  its  destined  uses.  The  animal  membrane,  which, 
as  we  have  seen,  is  the  bed  in  which  the  calcareous  phos- 
phate is  deposited,  partakes  of  the  reticular  structure  belong- 
ino-  to  the  ordinary  cellular  texture;  and  a  bone,  when  mi- 
nutely  examined,  exhibits  also  the  same  appearance  of  plates 
intermixed  vvith  fibres.  In  the  outer  compact  portion,  in-^ 
deed,  the  fibrous  arrangement  of  the  particles  is  not  so  easi- 
ly distinguished:  but  it  may  be  detected  in  young  bones 
while  they  are  becoming  ossified:  and  also  in  bones  that 
have  been  long  exposed  to  the  weather,  or  long  macerated 
in  water.  The  interior  of  most  bones,  in  the  higher  classes 
of  animals,  presents  distinctly  the  appearance  of  irregular 
cavities,  resulting  from  the  partial  separation  of  the  plates, 
and  their  mutual  crossings,  and  fibrous  connexions. 

The  different  mechanical  purposes  for  which  bones  are 
employed  in  the  animal  economy  require  them  to  be  of  dif- 
ferent forms.  Where  a  part  is  intended  to  have  compact- 
ness and  strength,  with  a  very  limited  degree  of  motion,  it 
is  divided  into  a  great  number  of  small  pieces,  united  toge- 
ther by  ligaments,  and  the  separate  bones  are  short  and  com- 
pressed, approaching  more  or  less  to  a  cubical  shape.  Of 
such  is  the  column  of  the  spine  composed,  as  also  the  joints 
of  the  wrist  and  ankle.  Where  the  principal  object  is  either 
extensive  protection,  or  the  provision  of  broad  surfaces  for 
the  attachment  of  muscles,  we  find  the  osseous  structure  ex- 


STRUCTURE  OF  BONE.  2G1 


pjtndcd  into  flat  plates;  as  is  exemplified  in  the  hones  of  the 
skull,  in  the  shoulder  hlade,  and  still  more  remarkahly  in  the 
bony  shield  which  surrounds  the  body  of  the  tortoise.  On 
the  other  hand,  where  a  system  of  levers  is  wanted,  as  in 
the  limbs,  which  have  to  sustain  the  weight  of  the  trunk, 
and  to  confer  extensive  powers  of  locomotion,  the  honcS  arc 
modelled  into  lengthened  cylinders,  generally  somewhat  ex- 
panded at  the  extremities,  for  greater  convenience  of  mutual 
connexion. 

In  the  form,  the  structure,  and  the  arrangement  of  these 
levers,  which  allow  of  the  regular  and  accurate  application 
of  the  moving  power,  and  are  calculated,  in  circumstances 
so  various,  to  give  effectual  support  to  the  fabric,  and  also 
to  execute  a  great  diversity  of  movements,  we  discern  most 
palpable  manifestations  of  profound  design,  and  the  most 
exquisite  refinements  of  mechanic  skill.  All  the  scientific 
principles  of  architecture  and  of  dynamics  are  more  or  less 
exemplified  in  the  construction  of  this  part  of  the  animal 
fabric.  Levers  of  various  kinds  are  most  artificially  com- 
bined in  the  formation  of  the  fins  of  fishes,  the  wines  of 
birds,  and  the  limbs  of  quadrupeds.  The  power  of  the  arch 
in  resisting  superincumbent  pressure  is  exhibited  in  various 
parts  of  the  osseous  systems  of  vertebrated  animals;  such  as 
the  human  foot,  the  spine,  the  pelvis,  and  more  especially 
in  the  vaulted  roof  of  the  skull,  and  in  the  carapace,  or  upper 
shell,  of  the  tortoise. 

The  construction  of  these  levers  evinces  that  a  minute  at- 
tention has  been  bestowed  on  every  condition  by  whicli  me- 
chanical advantage  could  be  gained.  In  the  more  perfect  de- 
velopments of  structures,  such  as  those  which  oi)tain  in  the 
higher  orders  of  mammalia,  and  also  in  the  class  of  birds,  all 
the  long  bones  are  hollow  cylinders,  and  their  cavity  is 
largest  in  the  middle  of  their  length.  This  is  shown  in  Fig. 
172,  which  represents  a  longitudinal  section  of  a  human 
thigh  bone,  and  in  Fig.  173,  which  is  a  similar  section  of 
the  humerus,  or  bone  of  the  arm.  The  walls  of  these  bones 
consist  of  a  dense  and  compact  substance,  formed  by  the 
close  coliesion  of  the  osseous  plates.     These  walls  arc  of 


262 


THE  MECHANICAL  FUNCTIONS. 


greater  thickness  in  the  middle  of  the  shank  or  shaft  of  the 
column,  and  hecome  thinner  as  we  follow  them  towards 

either  of  the  ends.  This  gradual  diminu- 
tion in  the  thickness  of  the  walls  arises 
from  the  continual  separation  of  the  plates, 
which  bend  inwards,  and  crossing  each 
other,  leave  a  multitude  of  irregular 
spaces  or  cells,  which  are  called  cancelli. 
The  plates,  proceeding  from  each  side 
obliquely  inwards,  at  length  meet  each 
other  in  the  axis  of  the  cylinder,  so  as  to 
close  the  middle  cavity  near  the  extremi- 
ties of  the  bone,  where  this  spongy  or  can- 
cellated structure  is  found  to  occupy  its 
whole  diameter. 

Now  if  we  consider  that  the  principal 
mechanical  property  required  in  every 
cylindrical  lever  is  rigidity,  and  more 
especially  the  power  of  resisting  forces 
applied  transversely,  that  is,  tending  to 
break  the  cylinder  across,  we  shall  soon 
perceive,  that  a  given  quantity  of  materials 
could  not  possibly  have  been  disposed  in  a  manner  better  cal- 
culated for  such  resistance  than  v/hen  in  the  form  of  a  tube, 
or  hollow  cylinder.*  To  this  mechanical  principle  I  have 
already  had  occasion  to  advert,  when  speaking  of  the  hollow 
stems  of  vegetables,  which  derive  their  chief  strength  from 
their  possessing  this  form;!  and  we  now  find  it  again  ap- 
plied in  the  structure  of  bones,  which  by  having  been  made 
hollow,  are  rendered  considerably  stronger  than  if  the  same 
materials  had  been  collected  into  a  solid  cylinder  of  the 
same  length.  We  may  farther  remark,  that  as  it  is  in  the 
middle  of  the  shaft  that  the  strain  is  greatest,  so  it  is  here 
that  the  cavity  is  largest,  and  the  resistance  most  effectual. 


*  An  elaborate  mathematical  demonstration  of  this  proposition  was  long 
ago  given  by  Dr.  Porterfiekl,  in  a  paper  contained  in  the  first  volume  of 
Medical  Essays  and  Observations,  publislied  by  a  Society  in  Edinburgh, 
p.  95. 

t  P.  70. 


OSSIFICATION.  263 


§  3.  Formation  and  Development  of  Bone. 

But  it  is  not  enough  to  contemplate  the  purposes  so  admi- 
rably answered  by  tliesc  arrangements.  Our  curiosity  can- 
"iiot  but  be  powerfully  excited  to  learn  what  processes  and  re- 
fined series  of  means  are  employed  by  nature  to  raise  and  to 
perfect  all  these  artificially  contrived  structures.  It  fortu- 
nately happens  that  in  this  instance  we  are  permitted  to 
penetrate  a  little  farther  than  usual  into  the  secrets  of  orsjanic 
evolution:  for  the  succession  of  changes  can  be  better  followed 
by  the  eye  in  the  slow  development  of  the  harder  parts,  than 
in  the  quicker  growth  of  mere  yielding  and  expansible  tex- 
tures. The  peculiar  material,  also,  of  which  bone  is  formed, 
is  easily  distinguished  by  its  hardness,  its  whiteness,  and 
its  opacity  from  the  softer  and  more  transparent  animal  sub- 
stance with  which  it  is  intermixed.  Hence  we  are  allowed 
an  opportunity  of  observing  the  earliest  stages  of  its  deposi- 
tion, and  of  accurately  following  the  subsequent  chano-cs  it 
undergoes.  • 

The  parts  of  the  embryo  animal,  which  are  destined  to 
become  bone,  partake  of  the  soft  and  gelatinous  consistence, 
which,  at  that  early  period,  characterizes  all  the  textures  of 
the  body;  and  they  can  hardly,  indeed,   be  distinguished 
from  the  semi-fluid  portions  which  surround  them.     In  pro- 
cess of  time,  when  tlic  vascular  circulation  of  the  blood  has 
been  established,  and  the  newly  formed  arteries  have  extend- 
ed their  branches  over  every  part  of  the  nascent  organization, 
those  vessels  which  are  appropriated  to  the  task  of  forming 
the  bones,  arrive  at  the  pulpy  masses  where  their  work  is 
to  commence.  As  sculptors,  before  working  upon  the  marble, 
first  execute  a  model  of  a  coarser  and  more  plastic  material, 
so  the  first  business  of  these  arteries  is  to  prepare  a  model 
of  the  future  bone,  constructed,  not  with  the  same  material 
of  which  it  is  afterwards  to  consist,  but  with  another  of  a 
simpler  and  softer  nature,  namely,  cartilage.    In  every  case, 
then,  cartilage  is  first  formed,  and  becomes  visible  by  its 
greater  opacity  when  conii)arcd  with  tiic  adjacent  jelly.     It 


264  THE  MECHANICAL  FUNCTIONS. 

is  an  exact  representation,  in  miniature,  of  the  bone,  which 
is,  in  due  course,  to  take  its  place.  It  is  evident  that  until 
the  other  parts  of  the  fabric  have  proceeded  so  far  in  their 
development  as  to  have  acquired  a  certain  degree  of  soli- 
dity and  firmness,  and  to  bear,  as  well  as  to  require,  the 
support  of  more  massive  and  rigid  structures,  this  flexible 
and  elastic  cartilage  may  be  employed  with  great  advantage 
as  its  substitute.  A  hard  and  unyielding  structure  would, 
in  the  early  stages  of  its  formation,  have  even  been  injuri- 
ous. But  in  proportion  as  the  fabric  is  enlarged,  the  ne- 
cessity for  mechanical  support  increases,  and  farther  provi- 
sion must  be  made  for  resistance  to  external  violence. 

When,  at  length,  all  is  prepared  for  the  construction  of 
the  bone,  the  next  step  to  be  taken  is  the  removal  of  the 
cartilage,  which  had  been  erected  as  the  scaffolding  for  the 
intended  building.     But  in  taking  down  this  scaffolding,  the 
whole  must  not  be  removed  at  once;  each  part  must  be  car- 
ried away,  piece  by  piece,  while  the  operation  of  fixing  in 
their  position  the  beams  and  pillars  of  the  edifice  proceeds. 
The  way  is  cleared  at  first  b}^  the  absorption  of  the  central 
part  of  the  cartilage,  and  a  few  particles  of  ossific  matter 
are  deposited  in  its  room.     While  this  process  is  going  on, 
greater  activity  is  displayed  in  the  arteries;  they  rapidly 
enlarge  in  diameter,  so  as  to  admit  the  colouring  globules 
of  the  blood;  and  they  thus  become  visible  to  the  eye,  which 
can  now  follow  their  course  without  difficulty.     From  being 
at  first  red  points,  they  soon  spread  out  into  lines,  of  which 
we  trace  the  branches  to  a  certain  extent,  although  we  can- 
not pursue  them  to  their  minuter  ramifications.     They  now 
assume  more  active  functions,  and  hasten  to  execute  their 
task  by  depositing  granules  of  calcareous  phosphate:  these 
are  laid  down,  particle  by  particle,  in  a  certain  determinate 
order,  and  in  regular  lines,  so  as  to  foWn  continuous  fibres. 
When  a  great  number  of  these  delicate  fibres  are  gathered 
together,  and  connected  by  other  fibres,  which  shoot  in  va- 
rious directions  across  them,  a  texture  composed  of  an  as- 
semblage of  long  spicula,  or  thin  plates,  is  constituted. 
In  the  cylindrical  bones,  the  spicula  prevail,  and  they  are 


OSSIFICATION. 


265 


arranged  longitudinally,  and  parallel  to  one  another,  and  to 
the  axis  of  the  bone.  They  first  constitute  a  ring  in  the 
middle  of  its  length:  this  ring  enlarges  in  all  its  dimensions, 
but  principally  in  its  length;  the  spicula  becoming  larger, 
not  by  the  stretching  of  their  parts,  in  consequence  of  the 
insinuation  of  fresh  materials  between  those  already  depo- 
sited, but  by  the  addition  of  new  particles  at  both  their  ex- 
tremities. In  like  manner,  the  ring  increases  in  thickness, 
not  by  the  deposition  of  phosphate  of  lime  between  the  ori- 
ginal layers,  but  by  the  application  of  fresh  layers  on  the 
outside  of  those  already  existing. 

In  the  flat  bones,  the  process  of  ossification  is  very  simi- 
lar to  what  I  have  just  described;  only  the  fibres  have  a  ra- 
diated arrangement,  shooting  out  from  the  spot  where  the 
first  deposite  took  place,  as  from  a  common  centre.  This  is 
seen  in  Fig.  174,  which  represents  the  parietal  bone  of  the 

175  iT/i  175 


human  skull,  in  an  early  stage  of  its  ossification,  and  shows 
the  radiating  fibres  very  distinctly.  In  the  cubical,  and 
more  irregularly  shaped  bones,  the  process  is,  doubtless, 
conducted  with  the  same  order  and  regularity,  although  it 
cannot  so  readily  be  followed  by  the  eye. 

The  same  process  is  repeated  in  different  parts  of  the  bone, 
wherever  nature  has,  in  conformity  with  determinate  laws 
of  development,  appointed  particular  centres  of  ossification. 
The  bone  continues  to  extend  from  each  of  these  centres, 
proceeding  gradually  towards  the  circumference,  or  the  re- 
moter parts  of  the  cartilage,  on  which  the  ossific  materials 
are  moulded,  and  by  the  form  of  which  that  of  the  future 

Vol.  I.  34 


266  THE  MECHANICAL  FUNCTIONS. 

bone  is  regulated.  The  process  of  ossification  has,  however, 
this  peculiarity,  that  the  cartilage  is  progressively  absorbed 
to  make  room  for  the  deposites  of  bony  substance.  When, 
the  bone  is  long,  separate  points  of  ossification  appear  in  the 
extremities,  before  the  central  portions  are  ossified;  and  the 
ends,  thus  formed  into  bone,  are  afterwards  united  to  the 
shaft,  so  that  the  whole  shall  form  a  continuous  bony  mass. 
In  the  flat  bones,  also,  if  the  surface  be  extensive,  an  addi- 
tional number  of  arteries  are  engaged  to  perform  the  work, 
which  is  begun  from  several  auxiliary  centres  of  ossification, 
and  the  completion  of  which  is  materially  accelerated  by 
their  co-operation. 

This  mode  of  increase  often  gives  rise  to  a  curious  result, 
of  which  a  striking  example  is  presented  in  the  bones  of  the 
skull.  The  brain,  which  these  bones  are  designed  to  pro- 
tect, requires  their  protection  at  a  very  early  period  of  life. 
The  growth  of  so  large  a  surface  of  bone,  as  would  be  re- 
quired for  covering  the  brain,  could  not  have  proceeded 
with  sufficient  quickness  for  the  exigencies  of  the  occasion, 
if  it  had  originated  from  a  single  point.  Therefore  it  is 
that,  besides  being  commenced  at  a  very  early  age,  the  pro- 
cess goes  on  from  a  great  number  of  separate  points  at  the 
same  time.  The  ossification  is  evidently  hurried  on  in  order 
to  complete  the  roofing  in  of  the  edifice  by  the  time  at  which 
the  animal  is  to  be  ushered  into  the  world,  and  exposed  to 
dangers  from  the  contact  of  external  bodies.  The  divergent 
fibres  shoot  out  rapidly,  coalescing  with  those  in  their  im- 
mediate neighbourhood,  which  co-operate  to  form  an  exten- 
sive bony  plate.  When  they  have  reached  the  prescribed 
line,  they  have  become  so  much  expanded  as  to  have  lost 
the  power  of  coalescing  with  the  fibres  which  have  origi- 
nated from  other  centres,  and  are  proceeding  in  a  contrary 
direction.  Yet  the  arteries  still  continuing  to  deposite  ossific 
matter,  each  set  of  fibres  insinuate  themselves  between  those 
of  the  opposite  set,  for  some  little  distance,  and  until  their 
farther  progress  is  stopped  by  the  increasing  resistance  they 
encounter.  The  consequence  is  that  the  edges  of  the  bones, 
which  have  thus  met,  are  irregularly  jagged,  like  the  teeth 


OSSIFICATION.  267 

of  a  saw,  presenting  externally  the  zig-zag  line  of  junction 
which  is  called  a  sulicre.  This  is  seen  in  Figures  175  and 
176,  the  former  of  which  represents  the  upper  side  of  the 
skull  of  an  infant;  and  the  latter,  the  same  bones  when  com- 
pletely ossified. 

The  union  of  bony  fibres  proceeding  from  different  cen- 
tres of  ossification  is  not  indiscriminate,  but  is  found  to  be 
regulated  by  definite  laws,  and  to  have  certain  relations  to 
the  general  plan  of  conformation  originally  established. 
Each  distinct  bone  is  formed  from  a  certain  number  of  ossi- 
fic  centres,  which  altogether  constitute  a  system  appertain- 
ing to  that  bone  only,  and  not  extending  to  the  adjacent 
bones.  These  pieces  unite  together,  as  if  by  a  natural  affi- 
nity; and  they  refuse  to  unite  with  the  bony  fibres  proceed-- 
ing  from  neighbouring  centres,  and  belonging  to  other 
groups.  The  groups  themselves  are  not  arbitrary,  but  are 
pre-established  parts  of  the  original  design.  Circumstances 
occasionally,  indeed,  arise,  which  may  overrule  this  inhe- 
rent tendency  to  preserve  the  line  of  separation  between 
two  bones;  and  we  then  fiind  them  coalescing  to  form  a  sin- 
gle piece.     Such  unions  are  technically  called  anchyloses. 

Were  this  the  whole  of  what  takes  place  in  the  formation 
of  a  bone,  the  process  would  not,  perha])s,  differ  very  mate- 
rially from  that  by  which  a  shell  is  produced;  for  a  shell, 
as  we  have  seen,  is  the  result  of  successive  depositions  of 
calcareous  matter,  forming  one  layer  after  another,  in  union 
with  a  corresponding  deposite  of  animal  membrane.  But  the 
subsequent  changes  which  occur,  show  that  the  constitution 
of  bone  is  totally  dissimilar  to  that  of  shell:  for  no  portion 
of  the  shell  that  is  once  formed,  and  has  not  been  removed, 
is  subject  to  any  farther  alteration.  It  is  a  dead,  thougli  per- 
haps not  wholly  inorganic  mass;  appended,  indeed,  to  the 
living  system,  but  placed  beyond  the  sphere  of  its  influence. 
But  a  bone  continues,  during  the  whole  of  life,  to  be  an  in- 
tegrant part  of  the  system,  partaking  of  its  changes,  modi- 
fied by  its  powers,  and  undergoing  continual  alterations  of 
shape,  and  even  renewals  of  its  substance,  by  the  actions  of 
the  living  vessels. 


268  THE  MECHANICAL  FUNCTIONS. 

The  form  which  had  at  first  been  rudely  sketched,  slow- 
ly advances  towards  perfection  in  the  course  of  its  growth; 
and  the  general  proportions  of  the  parts  are  still  preserved; 
the  finished  bone  exhibiting  prominences  and  depressions  in 
the  same  relative  situation  as  at  first;  and  not  only  having  si- 
milar internal  cavities,  but  being  frequently  excavated  in  parts 
which  had  before  been  solid.  During  all  these  gradual  altera- 
tions of  shape,  however,  there  is  no  stretching  of  elastic  parts; 
for  all  the  osseous  fibres  and  laminse  are  rigid  and  unyield- 
ing, and  in  this  respect  retain  an  analogy  with  shell.  The 
changes  thus  observed  can  have  been  effected  in  no  other 
way  than  by  the  actual  removal  of  such  parts  of  the  young 
bone  as  had  occupied  the  situations  where  vacuities  are  found 
to  exist  in  the  old  bone.  We  find,  for  instance,  that  in  the 
early  state  of  a  bone  there  are  no  internal  cavities,  but  the 
whole  is  a  uniform  solid  mass.  At  a  certain  stage  of  ossifi- 
cation cells  are  excavated  by  the  action  of  the  absorbent  ves- 
sels, which  carry  away  portions  of  bony  matter  lying  in  the 
axis  of  the  cylindrical  or  in  the  middle  layer  of  the  flat  bones.* 
Their  place  is  supplied  by  an  oily  matter,  which  is  the  mar- 
row, as  the  growth  proceeds,  while  new  layers  are  deposited 
on  the  outside  of  the  bone,  and  at  the  ends  of  the  long  fibres, 
the  internal  layers  near  the  centre  are  removed  by  the  ab- 
sorbent vessels,  so  that  the  cavity  is  farther  enlarged.  In 
this  manner  the  outermost  layer  of  the  young  bone  gradual- 
ly changes  its  relative  situation,  becoming  more  and  more 
deeply  buried  by  the  new  layers  which  are  successfully  de- 
posited, and  which  cover  and  surround  it;  until  by  the  re- 
moval of  all  the  layers  situated  nearer  to  the  centre,  it  be- 
comes the  innermost  layer;  and  is  itself  destined  in  its  turn 
to  disappear,  leaving  the  new  bone  without  a  single  particle 
which  had  entered  into  the  composition  of  the  original  struc- 
ture. 

It  has  been  found  that  by  mixing  certain  colouring  sub- 

•  The  bones  of  the  lower  classes  of  vertebrated  animals,  as  of  Fishes  and 
Reptiles,  seldom  reach  this  stage  of  ossification,  but  remain  solid  throughout; 
corresponding  to  the  bones  of  the  higher  classes  at  the  early  periods  of  their 
development. 


SKELETON  OF  THE  VERTEBRATA.  209 

stances  wllh  the  food  of  animals  the  bones  will  soon  become 
deeply  tinged  by  them.  This  fact  was  discovered  acciden- 
tally by  Mr.  Belchier,  who  gives  the  following  account  of 
the  circumstances  that  led  him  to  notice  it.*  Happening 
to  be  dining  with  a  calico  printer  on  a  leg  of  fresh  pork,  he 
was  surprised  to  observe  that  the  bones,  instead  of  being 
white  as  usual,  were  of  a  deep  red  colour;  and  on  inquiring 
into  the  circumstances,  he  learned  that  the  pig  had  been  fed 
upon  the  refuse  of  the  dying-vats,  which  contained  a  large 
quantity  of  the  colouring  substance  of  madder.  So  curious 
a  fact  naturally  attracted  a  good  deal  of  attention  among 
physiologists,  and  many  experiments  were  undertaken  to 
ascertain  the  time  required  to  produce  this  change,  and  to  de- 
termine whether  the  effect  was  permanent  or  only  temporary. 
The  red  tinge  was  found  to  be  communicated  much  more 
quickly  to  the  bones  of  growing  animals  than  to  those  which 
had  already  attained  their  full  size.  Thus  the  bones  of  a 
young  pigeon  were  tinged  of  a  rose  colour  in  twenty-four 
hours,  and  of  a  deep  scarlet  in  three  days;  while  in  the  adult 
bird,  fifteen  days  were  required  merely  to  produce  the  rose 
colour.  The  dye  was  more  intense  in  the  solid  parts  of 
those  bones  which  were  nearest  to  the  centre  of  circulation, 
while  in  bones  of  equal  solidity,  but  more  remote  from  the 
heart,  the  tinge  was  fainter.  The  bone  was  of  a  deeper  dye 
in  proportion  to  the  length  of  time  the  animal  had  been  fed 
upon  the  madder.  When  this  diet  was  discontinued,  the 
colour  became  gradually  more  faint,  till  it  entirely  disap- 
peared. I  shall  have  occasion  in  the  sequel,  to  discuss  the 
inferences  which  have  been  drawn  from  these  curious  facts. 

§4.    Skeleton  of  the  Vertchrata. 

The  purposes  to  be  answered  by  the  Skeleton,  in  vertc- 
brated  animals,  resolve  themselves  into  the  three  following; 
first,  the  affording  mechanical  support  to  the  body  generally, 
and  also  to  different  portions  of  the  body;  secondly,  the  pro- 

*  Philosophical  TransucUons  for  1736,  vol.  xxxix.  287  and  289. 


279  THE  MECHANICAL  FUNCTIONS. 

viding  a  solid  basis  for  the  attachments  of  the  muscles  which 
are  to  effect  their  movements;  and,  thirdly,  the  giving  pro- 
tection to  the  vital  organs,  but  more  particularly  to  the  cen- 
tral parts  of  the  nervous  system.  Of  these,  the  last  is  the 
circumstance  that  has  the  greatest  influence  in  determining 
the  principles  on  which  the  osseous  frame-work  has  been 
constructed.  In  the  nervous  system  of  all  the  animals 
coming  under  the  denomination  of  vertebrata,  the  spinal 
marrow,  together  with  the  brain,  which  may,  indeed,  be 
considered  as  the  anterior  extremity  of  the  spinal  marrow, 
only  much  enlarged  by  an  additional  mass  of  nervous  sub- 
stance, are  the  most  important  parts  of  that  sj^stem,  and  the 
organs  which  stand  most  in  need  of  protection  from  every 
kind  of  injury.  These  two  portions  of  the  nervous  system, 
when  viewed  as  composing  a  single  organ,  have  been  deno- 
minated the  spi)io -cerebral  axis,  in  contradistinction  to  the 
analogous  parts  of  the  nervous  system  of  articulated  animals: 
for,  amidst  great  differences  of  structure  and  of  functions,  an 
analogy  is  still  retained  among  the  several  forms  of  the  ner- 
vous system,  characterizing  these  two  great  divisions  of  the 
animal  kingdom.  In  the  embryo  state  of  the  vertebrata,  the 
central  parts  of  that  system  consist  of  two  separate  filaments, 
running  parallel  to  each  other  the  whole  length  of  the  body: 
but,  in  process  of  time,  these  two  filaments  unite,  and  con- 
stitute a  single  spinal  cord:  and  the  primary  type  of  the  ske- 
leton is  determined  by  the  peculiar  form  of  this,  the  central 
organ  of  the  nervous  system. 

In  laying  the  foundations  of  the  skeleton,  then,  the  first 
object  is  to  provide  for  the  security  of  the  spinal  cord:  and 
this  is  accomplished  by  enclosing  it  within  a  series  of  carti- 
laginous rings,  which  are  destined  to  shield  it  during  its 
growth,  and,  by  their  subsequent  ossification,  to  protect  it, 
most  effectually,  from  all  injurious  pressure.  It  is  this  part 
of  the  skeleton,  accordingly,  of  which  the  rudiments  appear 
the  earliest  in  the  embryo  animal.  These  rings  form  a  co- 
lumn, extending,  in  a  longitudinal  direction,  along  the  trunk; 
retracing  to  us  the  series  of  horny  rings,  in  which  the  bodies 
of  worms,  of  insects,  and,  indeed,  of  all  the  %/irticulata^  are 


VERTEBRAL  COLUMN. 


271 


incased.  When  ossified,  these  several  rings  arc  termed  vo'- 
Uhrx;  and  the  entire  column  which  they  compose  is  the 
Spine.  Fig.  177  shows  the  form  of  one  of  the  verteb^  of 
the  back  in  the  human  skeleton.  Fig.  178  is  a  side  view  of 
four  vertebrae  joined  together,  and  Fig.  179  is  a  vertical  sec- 


tion of  the  same  part  of  the  spine,  showing  the  canal  formed 
by  the  rings.  From  the  constancy  with  which  the  spinal 
column  is  found  in  all  animals  of  this  type,  and  from  the 
uniformity  of  the  plan  on  which,  amidst  endless  variations, 
it  is  modelled,  it  has  been  chosen  as  the  distinctive  charac- 
ter of  this  great  assemblage  of  animals,  which  have,  accord- 
ingly, been  denominated  the  Verlebrata,  or  Vcrtcbratcd 
Jinimals, 

Nor  is  the  spine  of  less  importance  when  viewed  in  its 
mechanical  relations  to  the  rest  of  the  skeleton.  It  is  the 
great  central  beam  of  the  fabric,  establishing  points  of  union 
between  all  its  parts,  and  combining  them  into  one  conti- 
nuous frame-work:  it  is  the  o;eneral  axis  of  all  their  motions, 
the  common  fulcrum  on  which  the  principal  bones  of  the 
extremities  are  made  to  turn:  it  furnishes  lixed  points  of  at- 
tachment to  all  the  large  muscles  which  act  upon  these  bones 
as  levers,  and,  also,  to  those  which  move  the  trunk  itself. 

If  this  column  had  been  perfectly  rigid,  the  whole  frame- 
work would  Ti^'C  been  exposed  to  inconvenience  and  even 
danger,  amidst  the  shocks  it  must  encounter  during  all  tlic 
quick  and  sudden  movements  of  the  body.  Not  only  must 
its  mechanism  be  framed  to  sustain  these  shocks,  but  also  to 


272  THE  MECHANICAL  FUNCTIONS. 

accommodate  itself  to  various  kinds  of  flexions,  and  twist- 
ings  of  the  trunk.  While  these  objects  are  provided  for, 
car^ust  at  the  same  time  be  taken  that  the  spinal  marrow 
it  encloses  shall,  amidst  all  these  motions,  remain  secure  from 
pressure;  for  so  delicate  is  its  structure  that  the  least  degree 
of  compression  would  at  once  interrupt  its  functions,  and 
lead  to  the  most  fatal  consequences.  A  safe  passage  is  like- 
wise to  be  afforded  to  the  nerves,  which  issue  from  the  spi- 
nal marrow,  at  certain  intervals,  on  each  side  throughout 
its  whole  length. 

No  where  has  mechanical  art  been  more  conspicuously 
displayed  than  in  the  construction  of  a  fabric  capable  of  ful- 
filling these  opposite,  and  apparently  incompatible  functions. 
The  principal  difficulty  was  to  combine  great  strength  with 
sufficient  flexibility.  This  we  find  accomplished,  first,  by 
the  division -of  the  column  into  a  great  number  of  pieces, 
each  of  which  being  locked  in  with  the  two  adjoining 
pieces,  and  tightly  braced  by  connecting  ligaments,  is  al- 
lowed but  a  very  small  degree  of  flexion  at  the  point  of  junc- 
tion. This  slight  flexion  at  each  single  joint,  however,  by 
becomino-  multiplied  along  the  series,  amounts  to  a  consider- 
able degree  of  motion  in  the  whole  column. 

The  broad  basis  of  each  bone  is  connected  with  the  next, 
not  by  a  joint,  but  by  a  plate  of  equal  breadth  (m,  m,  Figures 
178  and  179,)  composed  of  a  peculiar  substance,  intermediate 
in  its  texture  to  ligament  and  cartilage,  and  possessing  in  a 
remarkable  degree  the  qualities  of  toughness  and  adhesion, 
united  with  compressibility  and  elasticity.  By  yielding  for 
a  certain  extent  to  a  force  tending  to  bend  it  to  either  side, 
it  diminishes  the  quantity  of  motion  which  would  otherwise 
have  been  required  in  each  individual  joint:  and  by  acting  at 
the  same  time  as  a  spring,  it  softens  all  the  jars  and  -con- 
cussions incident  to  violent  action:  for  we  find  that,  how- 
ever the  spine  may  be  bent,  no  chasm  is  lefLby  the  flexions 
of  the  vertebrae  upon  one  another,  nor  is  tlie  continuity  of 
the  column  in  the  smallest  degree  interrupted. 

The  motions  of  the  vertebrae  upon  each  other  are  farther 
regulated  by  the  mode  in  which  their  articular  processes, 


VERTEBRAL  COLUMN.  273 

which  are  the  pieces  that  project  ohllquely  on  each  side, 
play  upon  each  other.  These  processes,  which  are  seen  at 
A,  A,  in  the  preceding  figures  (177  and  178)  arc  of  great  use 
in  preventing  the  sudden  displacement  of  the  vertebrae;  for 
this  effect  cannot  be  produced  by  any  force  short  of  that 
which  would  occasion  fracture.  Any  one  who  will  try  to 
dislocate,  by  sheer  force,  the  spine  of  a  hare  or  rabbit  will 
find  reason  to  admire  the  art  with  which  its  bones  have  been 
locked  together,  and  the  skill  displayed  in  combining  great 
flexibility  with  such  powerful  resistance  to  every  effort  that 
can  be  made  to  separate  them. 

For  the  purpose  of  allow^ing  a  passage  to  the  spinal  mar- 
row, tlie  bodies  of  the  vertebrae  (b,  Fig.  177  and  178,)  are 
hollowed  out  behind,  into  a  groove,  over  which  a  broad 
plate  of  bone  is  thrown  from  the  sides  of  the  vertebrae,  like 
the  arch  of  a  bridge.  The  succession  of  arches,  when  the 
vertebrae  are  joined  together,  forms  a  continuous  canal, 
which  is  occupied  by  the  spinal  marrow.  Notches,  corre- 
sponding to  each  other,  are  left  in  the  sides  of  each  of  the 
arches,  forming  apertures  for  the  secure  passage  of  the  nerves 
as  they  issue  from  the  spinal  marrow.  All  these  circum- 
stances are  visible  in  the  figures,  particularly  in  the  section. 
Fig.  179,  where  c,  c,  is  the  canal  for  the  spinal  marrow,  and 
in  which  the  apertures  just  mentioned  are  distinctly  seen,  at 

o,  o. 

In  order  to  give  an  advantageous  purchase  to  the  muscles 
which  are  attached  to  the  spine,  each  vertebra  lias,  besides 
the  parts  above  described,  a  projecting  piece  of  bone,  ex- 
tending upwards  from  the  crown  of  the  arch,  and  denomi- 
nated the  spinous  process  (s,  s.)  The  sharp  ridge  that  runs 
along  the  middle  of  the  back  of  a  quadruped,  is  formed  by 
the  continued  series  of  these  processes.  There  are  also,  on 
the  sides  of  the  vertebraj,  tw^o  other  projecting  pieces,  which 
are  denominated  the  transverse  processes  (t,)  and  which 
serve  as  levers  for  bending  the  column  laterally,  that  is,  ei- 
ther to  the  right  or  to  the  left.  All  these  com])onent  parts 
of  the  spine  are  subject  to  considerable  modifications,  in  dif- 
ferent tribes  of  animals,  according  to  the  particular  mccha- 

VoL.  I.  35 


274 


THE   MECHANICAL  FUNCTIONS. 


nical  circumstances  of  the  system,  and  to  the  particular  in- 
tentions of  their  formation. 

There  is  scarcely  any  part  of  the  osseous  fabric  of  which 
the  variations  better  illustrate  the  strict  unity  of  plan  and 
the  beautiful  law  of  gradation  observed  by  nature  in  all  her 
operations,  than  the  spine.  In  studying  the  various  modifi- 
cations which  this  part  of  the  skeleton  undergoes,  it  will  be 
useful  to  bear  in  mind  the  principles  which  appear  to  regu- 
late its  formation,  and  which  Geoffroy  St.  Hilaire  has  de- 
duced by  following  the  history  of  its  early  growth,  and  no- 
ticing the  order  in  which  its  several  parts  are  developed.* 
In  common  wath  all  bones,  the  vertebrae  take  their  rise  from 
certain  determinate  points,  or  centres  of  ossification,  where, 
at  first,  detached  pieces  of  bone  are  formed,  destined  to 
unite  together  so  as  to  compose  the  entire  bone.  An  accu- 
rate knowledge  of  the  general  forms  and  relative  situations 
of  these  elementary  pieces  is  of  much  importance,  because 
we  find  that  particular  circumstances  determine  the  deve- 
lopment of  some  of  these  parts  much  earlier,  and  to  a  greater 
extent  than  other  parts,  and  thus  lead  to  great  differences  in 
the  shapes  and  proportions  of  various  bones,  at  different  pe- 
riods of  their  growth,  although  their  origin  and  composition 
are  essentially  the  same. 

The  number  of  elements  which  enter  into  the  composi- 
tion of  a  vertebra  has  been  differently  estimated  by  different 

physiologists:  but  the  following  are 
certainly  entitled  to  that  character. 
They  are  represented  in  their  relative 
situations  in  Fig.  180.  The  first  is 
the  part  which  forms  the  nucleus,  or 
body  (b)  of  the  vertebra;  and  its  ossi- 
fication begins  at  the  centre.  Next 
in  importance  are  the  two  bony  plates, 
or  leaves,  as  they  may  be  called  (l, 
L,)  which  proceed  from  the  sides  of 
the  body,  and  embrace  the  spinal 
marrow  which  is  situated  between 
them.  The  fourth  essential  element 
*  Memoii'es  du  Museum,  ix.  79  and  89. 


STRUCTURE  OF  VERTEBRA.  275 

is  the  spinous  process,  (s,)  which  unites  the  two  leaves,  and 
thus  completes  the  superior  arch,  of  which  it  may  he  re- 
garded as  the  key  stone,  for  the  protection  of  the  spinal 
marrow.  Then  come  the  two  transverse  jyrocesses  (t,  t) 
which  extend  outwards  from  the  sides,  and  with  which  the 
arches  of  bone,  that  constitute  the  ribs  (r,  r,)  are  «renerally 
connected.  These  are  the  six  parts  which  may  be  consi- 
dered as  the  elements  that  are  most  essential,  and  most  con- 
stantly present  in  the  composition  of  the  vertebrae.  But 
some  other  parts  may  also  be  noticed  as  of  very  frequent 
occurrence:  such  are  the  bony  plates  which  cover  the  two 
flat  portions  of  the  bodies  of  the  vertebrae,  forming  the  sur- 
faces immediately  contiguous  to  the  intervertebral  ligament; 
which  surfaces,  in  some  of  the  lower  orders  of  the  verte- 
brata  become  articular.  There  is  frequently,  also,  a  deve- 
lopment of  processes,  (f,)  forming  arches  and  spines  at  the 
lower  surface  of  the  vertebrae,  or  the  one  opposite  to  that 
which  gives  rise  to  the  superior  arches  already  mentioned. 
This  structure  is  very  generally  met  with  in  fishes,  and  it 
is  observed  also  in  the  cetacea.  The  arches  thus  formed 
enclose  a  large  artery,  which  is  the  continuation  of  the  aor- 
ta, or  the  main  artery  running  along  the  back,  immediately 
under  the  spinal  column. 

There  are  still  other  processes,  less  constantly  present  and 
more  variable  in  their  shape.  They  form  articular  surfiices 
for  the  purpose  of  being  connected  with  the  surfaces  of  cor- 
responding processes  in  the  contiguous  vertebra.  Of  these 
there  are  four  (a,  a,  a,  a)  belonging  to  each  vertebra,  two 
in  front,  and  two  behind.  These,  however,  should  not  be 
included  among  the  primary  elements  of  the  vertebrae,  be- 
cause we  find  them,  in  different  instances,  occupying  differ- 
ent positions,  and  formed  sometimes  by  extensions  of  the 
bodies,  and  at  other  times  of  the  leaves.  In  following  tliem 
through  the  several  tribes  of  animals,  we  observe  them  shift- 
ing their  places,  in  various  ways,  and  not  even  preserving 
any  constancy  in  their  number.  They  are  wholly  absent  in 
fishes:  in  the  crocodile,  and  other  reptiles,  they  approximate 
so  as  to  form  three  articular  surfaces,  namely,  two  close  to 


276  THE  MECHANICAL  FUNCTIONS. 

one  another,  and  a  third  posterior  to  these.  In  the  Orni- 
thorhyncus, while  the  latter  retains  its  situation  in  the  mid- 
dle, the  other  surfaces  have  separated  from  each  other,  and 
have  travelled  outwards,  taking  their  stations  upon  the 
leaves.  In  the  Mammalia,  the  middle  surface  has  Avholly 
disappeared,  and  the  outer  surfaces  have  risen  into  what  are 
termed  the  oblique  processes. 

In  addition  to  these,  accessory  bones  are  often  developed 
to  suit  particular  occasions.  Thus,  in  fishes,  we  see  that  one 
or  two  additional  pieces  (i)  are  affixed  to  the  ends  of  each 
spinous  process.  In  many  cases,  instead  of  being  thus  placed 
in  a  line  with  these  processes,  they  appear  at  a  little  distance, 
as  if  they  had  slipped  from  their  proper  situations;  they  are 
then  found  between  the  spinous  processes,  and  receive  the 
name  of  interspinous  hoiies. 

The  spinous  processes  have  a  tendency,  when  their  de- 
velopment proceeds,  to  divide  into  two  branches,  and  this  bi- 
furcation frequently  takes  place  also  in  the  interspinous  bones. 
The  transverse  processes,  likewise,  occasionally  develope  ac- 
cessory pieces,  as  is  found  to  be  the  case  in  some  reptiles ; 
but,  in  other  instances,  they  undergo  a  gradual  change  of  po- 
sition, as  we  follow  them  backwards  along  the  spinal  column, 
where  they  descend  tow^ards  the  abdominal  region. 

The  flexibility  of  particular  portions  of  the  spinal  column 
is  regulated  by  the  size  and  form  of  its  processes.  When 
these  are  much  developed,  they  necessarily  obstruct  the  flex- 
ion of  the  vertebrae  in  the  directions  in  which  they  are  situ- 
ated :  when  they  are  small,  no  such  hinderance  arises,  and  the 
spine  is  free  to  move  in  all  directions.  Thus,  when  we  see 
the  spinous  processes  much  enlarged,  while  the  transverse 
processes  are  small,  we  may  infer  that  the  spine  is  incapable 
of  any  bending  in  that  direction ;  but  that  it  has  the  power  of 
free  lateral  flexion.  This  is  the  condition  of  the  spine  of 
fishes,  where  this  latter  kind  of  motion  is  the  one  principally 
wanted.  In  dolphins,  and  other  cetacea,  on  the  contrary, 
where  the  actions  are  required  to  be  vertically  upwards  and 
downwards,  the  spinous  processes  are  small,  and  the  trans- 
verse processes  very  long  and  broad. 


STRUCTURE  OF  THE  SPINE.  277 

Every  instance  of  variation  in  the  forms  of  these  impor- 
tant parts  of  the  osseous  system,  will,  in  like  manner,  be 
found  to  have  a  relation  to  some  particular  circumstance  in 
the  living  habits  of  the  animal,  and  to  be  subordinate  to  the 
general  plan  of  its  economy.  But,  in  order  to  understand 
the  mode  in  which  nature  has  effected  these  changes,  it  is 
necessary  to  study  the  elements  of  each  part  of  the  osseous 
system:  for  these  constitute  the  alphabet  by  which  the  com- 
binations she  presents  to  us  become  legible,  and  their  ori- 
gin and  jDrogress  are  unfolded  to  our  comprehension.  Ac- 
cording as  each  of  these  elements  of  ossification  receives  dif- 
ferent degrees  of  development,  so  do  the  different  bones 
they  compose  acquire  their  particular  shapes  and  relative  di- 
mensions. Sometimes,  indeed,  we  find  that  one  or  other  of 
these  elements  has  disappeared;  or,  at  least,  we  can  discover 
no  trace  of  its  development;  in  other  cases,  we  see  it  ex- 
ceedingly expanded,  and  appearing  under  forms  of  greater 
complication,  so  as  to  be  with  difficulty  identified:  on  some 
occasions,  as  we  have  just  seen  in  the  spinous  bones  of  fishes, 
its  accessory  structures  are  multiplied,  as  if  continued  eflbrts 
were  made  by  the  system  to  repeat  the  same  structures. 
Amidst  all  these  modifications,  the  parts  that  preserve  the 
greatest  constancy  of  form  are  those  which  are  of  most  im- 
portance, and  wdiich  are  constituent  parts  of  the  primordial 
type  of  the  class  to  which  the  individual  animal  belongs. 

The  spinal  column  is  generally  prolonged  at  its  posterior 
extremity  into  a  scries  of  vertebra},  which  are  sometimes 
exceedingly  numerous;  decreasing  in  their  size  as  they  ex- 
tend backwards,  and  having  continually  smaller  processes, 
the  one  disappearing  after  the  other,  till  all  of  them  are  lost, 
and  nothing  remains  in  those  at  the  extremity  of  the  series 
but  the  cylindrical  bodies  of  the  vertebrae.  Even  these  be- 
come stinted  in  their  growth  and  ossification,  until  w^e  find 
the  terminal  pieces  generally  remaining  in  the  state  of  car- 
tilage. Such  is  the  structure  of  the  osseous  support  of  the  tail, 
as  seen  in  many  quadrupeds  in  its  most  developed  forms.  It 
illustrates  the  law,  that  when  in  any  system  there  occurs  a 
frequent  repetition  of  the  same  structure,  the  evolution,  in 


278  THE  MECHANICAL  FUNCTIONS. 

the  latest  of  those  repetitions,  becomes  less  perfect,  and  ends 
by  being  abortive.  In  the  present  instance,  the  consequences 
of  this  law  are  highly  advantageous,  since  it  provides  for 
the  flexibility  of  the  tail,  and  qualifies  it  for  being  applied 
to  a  great  variety  of  useful  purposes,  as  we  find  more  espe- 
cially exemplified  in  the  Ateles,  or  spider  monkey,  and  in 
the  Kangiiroo, 

Next  in  importance  to  the  spine  is  the  craniuvi,  or  osse- 
ous covering  of  the  brain;  together  with  the  bones  of  the 
face,  which  protect  the  organs  of  the  finer  senses.  An  ac- 
curate investigation  of  the  mode  in  which  these  bones  are 
formed  has  led  many  modern  anatomists  to  the  opinion 
that  they  were  originally  parts  of  the  spinal  cokimn,  and 
that  they  are,  in  fact,  developments  of  vertebrae,  much  al- 
tered, indeed,  in  shape,  in  consequence  of  the  new  condi- 
tions to  which  they  have  been  subjected;  but  still  possessing 
all  the  essential  elements  of  vertebrae.  In  the  embryo  con- 
dition of  these  organs,  and  while  the  brain  is  yet  undeve- 
loped, the  resemblance  of  the  bony  circles  which  enclose  it 
to  vertebrae  is  certainly  very  striking;  but  in  proportion  as 
the  brain  becomes  expanded,  the  similarity  diminishes;  for 
the  rapid  growth  of  the  brain  in  the  higher  orders  of  animals 
is  necessarily  attended  with  an  equally  sudden  expansion  of 
the  bones  of  the  skull.  Hence,  their  several  elements  are 
thrown  into  unusual  positions,  and  being  variously  distorted 
and  disfigured,  can  hardly  be  recognised  under  the  strange 
disguises  they  assume. 

The  extensive  researches  that  have  been  recently  made  in 
this  branch  of  comparative  anatomy,  have  supplied  many 
facts,  which  tend  to  support  the  hypothesis  that  the  bony 
coverings  of  the  brain  are  the  result  of  the  development  of 
three  vertebrae.  According  to  this  theory,  the  first  of  these 
supposed  cranial  vertebrse,  beginning  our  enumeration  from 
the  neck,  is  the  origin  of  the  occipital  bone,  of  which  the 
lower  part,  or  that  which  immediately  supports  the  cerebel- 
lum, corresponds  to  the  body  of  the  vertebra;  the  two  lateral 
portions  to  the  leaves;  and  the  upper  flat  plate,  to  the  spinous 
process.     The  body  of  the  second  cranial  vertebra  becomes, 


SKELETON  OF  VERTEBRATA. 


279 


in  process  of  time,  the  posterior  half  of  the  sphenoid  bone, 
which  lies  in  the  middle  of  the  basis  of  the  skull;  the  tem- 
poral bones  being  formed  by  its  leaves,  and  the  parietal  bones 
by  the  lateral  halves  of  its  spinous  process.  The  third  cra- 
nial vertebra  is  constituted  by  the  anterior  half  of  the  sphe- 
noid bone,  which  is  its  body,  and  the  frontal  bones,  which 
are  its  leaves.  This  theory,  which  originated  with  Oken, 
has  been  farther  extended  to  the  bones  of  the  face,  by  Geof- 
froy  St.  Hilaire,  who  conceives  them  to  be  likewise  deve- 
lopments of  several  other  supposed  cranial  vertebrae;*  but 
the  analogies  by  which  the  hypothesis  is  supported  become 
more  feeble  and  confused,  as  we  recede  from  the  middle  of 
the  spinal  column. 

All  the  other  parts  of  the  skeleton  may  be  regarded  as  ac- 
cessory to  the  spine:  and  they  are  far  from  exhibiting  the 
same  constancy  either  in  form  or  number,  as  the  vertebral 
column.  In  some  instances,  as  in  serpents,  these  accessory 
parts  are  altogether  wanting;  in  others,  they  exist  only  in 
rudimental  states ;  and  it  is  but  in  a  few  that  they  can  be 
considered  as  having  reached  their  full  development.  In  or- 
der to  obtain  a  standard  of  comparison  by  which  to  estimate 
all  their  gradations  of  evolution,  it  will  be  best  to  consider 


*  In  this  theory  of  G.  St.  lliluire,  the  number  of  cranial  vci-tebra;  is  seven, 
each  composed  of  nine  elementary  pieces. 


280  THE    MECHANICAL  FUNCTIONS. 

them  first  in  their  more  perfectly  developed  forms,  as  they 
are  presented  in  the  higher  classes  of  quadrupeds.  In  the 
following  descriptions,  the  skeleton  of  the  Hog  (Fig.  181)  will 
be  taken  for  the  purpose  of  reference. 

The  ribs  consist  of  arches  of  bone  affixed  at  their  upper 
ends  to  the  bodies  of  the  vertebrae,  and  also,  by  a  separate 
articulation,  to  their  transverse  processes ;  where,  in  general, 
they  are  allowed  a  slight  degree  of  motion.  Their  primary 
use  is  to  defend  the  vital  organs  situated  in  the  region  of  the 
chest,  or  thorax,  (namely,  the  heart  and  the  lungs ;)  but  they 
are  subservient  also  to  the  function  of  respiration,  by  the  al- 
ternate movements  that  are  given  to  them  by  their  mus- 
cles. The  two  parts,  of  which  they  are  composed,  often  form 
an  angle  by  their  junction,  and  at  this  angle  a  process  occa- 
sionally extends,  for  the  purpose  of  forming  connexions  with 
the  neighbouring  ribs. 

The  ribs  are  connected  in  front  with  the  breast  bone,  or 
sternum  (s,)  often  by  the  intervention  of  cartilages,  which, 
from  their  similarity  of  form  to  the  ribs,  appear  as  continua- 
tions of  them,  and  are  provided  apparently  to  eke  out  the  re- 
mainder of  the  semicircle.  These  cartilages,  which  have 
been  termed  the  sterno-costal  appendices,  often  become  ossified 
either  wholly  or  in  part. 

The  sternum  is  formed  of  nine  elementary  pieces,  each  pro- 
ceeding from  a  separate  centre  of  ossification.  Two  of  these 
occupy  the  end  which  is  nearest  to  the  head,  four  are  lateral, 
and  two  are  situated  at  the  opposite  extremity  :  one  only  be- 
ing central  and  surrounded  by  the  rest.  Few  subjects  in 
comparative  osteology  are  more  curious  and  instructive  than 
to  trace  the  development  of  these  several  elementary  parts 
in  the  different  classes  of  animals,  from  the  rudimental  states 
of  this  bone  as  it  occurs  in  fishes,  to  its  greatly  expanded  con- 
ditions in  the  tortoise  and  the  bird,  wdiich  severally  exhibit 
the  most  opposite  proportions  of  these  elements. 

Last  in  the  order  of  constancy  come  the  bones  of  the  ex- 
tremities. As  we  ascend  in  the  scale  of  animals  we  may 
observe  the  prevalence  of  a  tendency  to  the  concentration 
of  organs,  and  consequently  to  the  diminution  of  their  num- 


SKELETO.V  OF  VERTERRATA.  281 

ber.     While  in  animals  of  llic  inferior  orders,  which  arc 
possessed  of  extremities,  we  find  a  considcrahle  number  of 
legs;  in  all  the  animals  comprised  in  the  class  of  true  insects 
nature  has  limited  the  number  to  six;  and  in  the  vertebrata 
it  never  exceeds  four.     As  in  insects,  we  observed  that  all 
the  legs  are  divided  into  the  same  number  of  parts,  so  we 
find  among  quadrupeds  a  striking   correspondence  in   the 
bones  of  the  fore  and  the  hind  extremities.     Both  the  one 
and  the  other  are  connected  with  the  spine  by  the  interme- 
dium of  large  and  broad  bones,  which  are  intended  to  serve 
as  a  basis  for  their  more  secure  attachment,  and  for  giving, 
at  the  same  time,  extensive  and  advantageous  purchase  to 
the  muscles,  which  are  to  move  the  limbs.     The  two  bones 
by  which   the  anterior   extremity   is   connected   with  the 
trunk  are  the  hlade-hone,  or  Scapula,  (u,)  which  sends  out 
a  process  called  the  coracoid  bone;  and  the  collar-hone,  or 
the  Clavicle,'^'  which  extends  from  the  scapula  to  the  ster- 
num.    The  corresponding  connecting  bones  of  the  posterior 
extremity  are  three  in  number,  and  constitute,  together  with 
the  part  of  the  spine  to  which  they  are  attached,  what  is 
called  the  Pelvis  (p.)     The  part  of  the  spine  which  is  thus 
included  in  the  pelvis,  is  termed  the  Sacrum.     In  its  com- 
plete state  of  ossification  it  is  a  single  bone;  but  it  was  ori- 
ginally composed  of  a  number  of  separate  vertebras,  which 
have  afterwards  become  consolidated  into  a  single  bone,  and 
which  bear  the  marks  of  having  been  compressed  from  be- 
hind forwards  during  their  growth,  so^that  they  could  only 
expand  laterally.     The  vertebrae  which  succeed  to  these, 
and  which  are  not  consolidated  with  the  sacrum,  compose 
what  is  called  the  os  coccygis,  (q,)  or  more  properly  the 
coccygeal  verlehrx:  when  they  are  sufliciently  numerous  to 
compose  a  tail,  they  come  under  the  denomination  of  caudal 
vertebrx.     The  three  bones  of  the  pelvis,  are  the  iliu7?i,  the 

•  This  bone  does  not  exist  in  the  skeleton  of  the  hog-:  but  its  form  and 
connexions  with  the  sternum  and  scapula,  in  the  liuman  skeleton,  arc  slio\sn 
in  Fig-.  182,  where  s  is  the  sternum;  c,  the  clavicle;  h,  the  scapula;  a,  the 
acromion;  k,  the  coracoid  process;  and  g-,  the  glenoid  cavity  for  the  articula- 
tion of  the  humerus. 

Vol.  I.  36 


282  THE  MECHANICAL  FUNCTIONS. 

ischhim,  and  the  pubis.  They  all  concur  in  the  formation 
of  a  large  cup-like  cavity,  called  the  aceiabidum,  which 
receives  the  head  of  the  thigh  bone  (r,)  constituting,  gene- 
rally, the  largest  joint  in  the  body. 

A  single  bone  composes  the  first  division  of  each  limb, 
both  in  the  fore  and  hind  extremities.  In  the  fore  leg  it  is 
termed  the  humerus  (n,)  in  the  hind  leg,  the  femur  (f.) 
The  next  division  contains  two  bones,  placed  parallel  to  each 
other;  they  are,  in  the  former,  the  radius  (r,)  and  the  ulna 
(u;)  in  the  latter,  the  tibia  (t,)  and  fibula  (f.)  These  are 
followed  by  a  number  of  small,  rounded,  or  cubical  bones, 
collected  together  in  a  group,  which  constitutes  the  Carpus 
(w,)  in  the  fore  leg,  and  the  Tarsus  (t,)  in  the  hind  leg. 
Next  come  a  set  of  long  cylindrical  bones,  composing  the 
metacarpus  (m,)  in  the  former,  and  the  metatarsus  (m,)  in 
the  latter  case.  In  the  most  complete  forms  of  development, 
these  are  always  five  in  number,  in  each  limb;  they  are 
placed  generally  parallel  to  each  other,  but  are  enveloped  in 
one  common  covering  of  integument.  The  Phalanges,  or 
toes  (z,)  are  cylindrical  bones,  continued  in  a  line  from  each 
of  the  former:  they  are  generally  three  in  number  in  each 
toe.  To  the  last  joint,  which  is  often  termed  the  ungual 
bone,  there  is  usually  attached  either  a  nail,  a  claw,  or  a 
hoof.  Small,  detached  bones  are  frequently  found  at  the  ex- 
terior part  of  the  angles  which  they  form  by  their  junction, 
serving  the  purpose  of  giving  a  more  advantageous  position 
to  the  tendons  of  tl\e  muscles  which  extend  those  joints. 
i:he  patella,  or  knee  pan  (k,)  is  the  largest  of  these,  and  is 
pretty  constantly  present.  Smaller  bones  of  this  description 
are  met  with  on  the  joints  of  the  fingers,  and  are  termed  ^e- 
samoid  bones. 

On  comparing  these  divisions  of  the  limbs  of  quadrupeds 
with  those  of  insects,  we  cannot  fail  to  perceive  that  there 
exists  between  them  a  marked  analogy;  and  that  naturalists 
were  not  led  away  by  mere  fancy  when  they  applied  to  the 
latter  the  same  names  as  those  borne  by  the  former.  This, 
however,  is  not  the  only  instance  of  analogy  that  may  be 
discovered  between  the  structures  of  articulated  and  of  ver- 


SKELETON  OF  VERTEIJRATA.  283 

tebratcd  animals,  however  strong  may  be  the  contrast  wliich 
they  offer  in  all  the  essential  features  of  their  conformation. 
The  rings  whicli  compose  the  skeleton  of  the  insect,  and 
which  enclose  its  principal  nervous  chords,  have  been  sup- 
posed to  have  an  analogy  with  the  circles  of  bone  which  con- 
stitute the  primary  forms  of  the  vertebrae,  and  which  con- 
lain  the  spinal  chord;  although,  in  the  first  case,  it  is  true, 
other  viscera  are  included  within  the  arches,  whereas,  none 
are  contained  in  the  last  case.  They  agree,  also,  in  having 
the  head  placed  at  one  extremity,  distinct  from  the  trunk, 
and  containing  the  principal  organs  of  the  senses.  Farther 
correspondences  have  been  likewise  traced  in  the  minuter 
anatomy  of  these  parts,  which  it  would  here  occupy  too 
much  space  to  examine  in  detail. 

An  approximation  is  evidently  made  towards  an  internal 
skeleton  in  the  cephalopodous  mollusca;  wlicre  we  find  a 
central  body,  cartilaginous  in  some  species,  calcareous  in 
others.  In  the  Loligo  it  has  a  long  and  slender  shape,  and 
is  pointed  at  the  end  like  the  blade  of  a  sword;  it  bears,  as 
we  shall  hereafter  notice,  some  resemblance  to  the  cartila- 
ginous spine  of  the  fish  called  the  Myxine,  or  Gasirobran- 
chus,  which  does  not  enclose  the  spinal  marrow,  but  only 
admits  it  to  pass  along  a  groove  in  its  upper  edge. 

All  these  multiplied  instances,  when  weighed  together, 
and  united  in  a  comprehesive  view,  are  sufiicient  to  prove, 
that  there  exist  very  perceptible  links  of  connexion  among 
all  the  classes  of  created  beings,  even  in  those  apparently 
the  most  remote  from  one  anotlier.  They  render  it  clear  to 
the  discerning  eye  of  the  philosophic  naturalist,  that  all  tiie 
races  of  animated  beings  are  members  of  one  family,  and  the 
offspring  of  the  same  provident  Parent,  who  has  matured  all 
his  plans  on  a  deeply  premeditated  system,  and  who  dis- 
penses all  his  gifts  with  the  most  salutary  regard  to  the  ge- 
neral welfare  of  his  creatures. 


(     284     ) 


CHAPTER  VII. 

FISHES. 

In  reviewing  the  series  of  animals  which  compose  each 
great  division  of  this  kingdom  of  nature,  we  constantly  find 
that  the  simplest  structures  and  modes  of  progression  are 
those  belonging  to  the  aquatic  tribes.  Among  vertebrated 
animals,  the  lowest  rank  is  occupied  b}^  Fishes,  a  class  com- 
prehending an  immense  number  of  species,  which  are  all 
inhabitants  of  the  water,  which  exhibit  an  endless  variety 
of  forms,  and  open  to  the  physiologist  a  wide  field  of  in- 
teresting research.  We  cannot  fail  to  perceive,  on  the  most 
cursory  glance,  the  beautiful  adaptation  of  the  form  and  struc- 
ture of  all  these  animals  to  the  properties  of  the  element  in 
which  they  are  destined  to  reside.  In  order  that  the  fish 
might  glide  through  the  fluid  with  the  least  resistance,  all 
its  vital  organs  have  been  collected  into  a  small  compass, 
and  the  body  has  been  reduced  into  the  shape  of  a  compact 
oval,  compressed  laterally:  and  tapering  to  a  thin  edge, both 
before  and  behind;  for  the  purpose  of  readily  cleaving  the 
water  as  the  fish  darts  forward,  and  also  of  obviating  the  re- 
tardation that  might  arise  from  the  reflux  of  the  water  col- 
lected behind.  With  a  view^  to  diminish  friction  as  much 
as  possible,  the  surface  of  the  body  has  been  rendered  smooth, 
and  the  skin  impregnated  with  oil,  which  defends  it  from 
injurious  impressions,  and  at  the  same  time  prevents  the 
water  from  penetrating  into  its  substance. 

The  body  of  a  fish  is  nearly  of  the  same  specific  gravity 
as  the  water  it  inhabits;  and  the  efiect  of  gravity  is  therefore 
almost  wholly  counterbalanced  by  the  buoyant  force  of  that 
fluid:  for  the  weight  of  a  mass  of  water,  equal  in  bulk  to  the 
body  itself,  is  the  exact  measure  of  this  buoyant  force.  If 
this  weight  w^ere  precisely  the  same  as  that  of  the  fish,  the 
animal  would  be  able  to  remain  suspended  in  any  pari  of  the 


FISHES.  285 

.fluid  without  the  necessity  of  employing  any  voluntary  mo- 
tion or  exertion  for  that  purpose:  but  as  the  body  of  a  fish 
is  generally  a  little  heavier  than  the  fluid  medium,  csj)ecial- 
ly  if  it  be  fresh  water,  it  is  necessary  for  the  animal  to  give 
its  body  some  degree  of  motion,  in  order  to  prevent  its 
sinking. 

In  land  quadrupeds,  the  limbs  have  to  perform  the  double 
office  of  supporting  the  body,  and  of  eficcting  at  the  same 
time  its  locomotion:  but  as  nearly  the  whole  of  the  weight 
of  a  fish  is  already  sustained  by  the  clement  in  which  it  is 
immersed,  its  instruments  of  motion  may  be  employed  ex- 
clusively for  progression,  and  the  powerful  hydrostatic  pres- 
sure, which  supports  the  body  on  all  sides,  supersedes  the 
necessity  of  that  cohesive  rigidity  of  frame,  which  is  essen- 
tial to  the  safety  of  terrestrial  animals.  Hence  we  find  that 
in  one  whole  tribe  of  fishes,  the  skeleton  is  composed  mere- 
ly of  cartilage;  and,  in  all,  it  exhibits  much  less  of  the  osse- 
ous character  than  in  the  higher  classes.  The  frame-work 
of  the  skeleton,  even  of  osseous  fishes,  has  not  the  compact- 
ness possessed  by  that  of  quadrupeds  or  reptiles:  the  pieces 
which  compose  it  are  joined  together  less  firmly;  many  of 
them,  indeed,  remain  in  an  imperfectly  ossified  condition, 
their  elementary  pieces  being  detached  from  one  andther,  as 
if  the  usual  process  of  consolidation  had  been  arrested  at  an 
early  stage.  The  texture  of  the  bones  of  cartilaginous  fishes 
corresponds  to  this  primeval  condition;  for  it  is  composed 
merely  of  granules  of  calcareous  phosphate,  interspersed 
amidst  the  cartilaginous  substance  in  detached  masses,  or 
presenting  the  appearance  of  coarse  fibres,  thinly  scattered 
through  the  semitransparent  bone.  Compared  with  the 
quantity  of  gelatin  which  enters  into  their  composition,  the 
bones  of  fishes  contain  but  a  small  proportion  of  earthy  in- 
gredient, a  circumstance  which  explains  the  pellucidity  of 
the  mass,  and  the  readiness  with  which  the  osseous  fibres  it 
contains  can  be  distinguished.  Another  consequence  of  the 
want  of  density  in  the  bones  of  fishes  is,  that  their  articula- 
tions are  less  regular  and  perfect  than  the  corresponding 


286 


THE  MECHANICAL  FUNCTIONS. 


joints  of  terrestrial  animals;  for  it  is  evident  that  where  the, 
parts  are  soft  and  flexible,  joints  are  not  required. 

In  the  osseous  fishes,  the  bony  structures  are  more  finished; 
and  they  even  arrive  at  a  degree  of  hardness,  equal  to  that 
of  the  higher  classes.  But  this  development  is  not  uniform  in 
all  the  bones;  in  the  head  of  the  pike,  for  instance,  while 
some  of  the  bones  have  acquired  a  great  hardness,  others  re- 
main wholly  and  permanently  in  a  cartilaginous  condition. 
The  bones  of  fishes,  however  advanced  in  their  ossification, 
never  reach  that  stage  of  the  process  in  wdiich  cavities  are 
formed ;  thus  there  is  no  space  for  marrow,  nor  even  for  the 
cellular  or  cancellated  structure  which  we  have  noticed  in 
the  more  perfect  bones.*     The  general  disposition  of  the 


bones  which  compose  the  entire  skeleton  will  be  understood 
from  Fig.  184,  w'hich  represents  that  of  the  Cijpriniis  carpio, 
or  carp.  The  muscular  flesh  of  fishes  is  likewise  softer  than 
that  of  the  higher  classes;  and  the  cellular  substance  more 
attenuated  and  more  gelatinous;  so  that  the  membranes 
which  it  forms  are  of  a  looser  and  m.ore  pulpy  texture. 

Progressive  motion  in  fishes  is  effected  by  the  simplest 
means,  the  principal  instrument  employed  for  this  purpose 
beina;  the  tail ;  for  the  fins,  as  we  shall  presently  find,  are 
merely  auxiliary  organs,  serving  chiefly  to  balance  the  body 
while  it  receives  its  propulsion  from  the  tail.  A  fish  moves 
in  the  water  upon  the  same  principle  as  a  boat  is  impelled 


Cuvier,  sur  les  Polssons.     Tom.  i.  p.  218. 


FISHES.  287 

in  sculling ;  for  the  action  of  the  tail  upon  the  \vatcr  is  late- 
ral, like  that  of  an  oar,  which  it  resembles  in  the  vertical  po- 
sition of  its  plane;  and  the  effect  is  transferred  by  the  resist- 
ance of  the  water  to  the  body  where  the 
impulse  originates.  Let  us  suppose,  for 
example,  that  the  tail  is  slightly  inclined 
to  the  right,  as  shown  in  Fig.  185.  If,  in 
this  situation,  the  muscles  on  the  left  side, 
tending  to  bring  the  tail  in  a  right  line 
with  the  body,  are  suddenly  thrown  into 
action,  the  resistance  of  the  water,  by  re- 
acting against  the  broad  surface  of  the 
tail  in  the  direction  p  r,  perpendicular  to 
that  surface,  will  cause  the  muscular  ac- 
tion to  give  the  whole  body  an  impulse  in  that  direction;  and 
the  centre  of  gravity,  c,  will  move  onwards  in  the  direction 
c  B,  parallel  to  p  r.  This  impulse  is  not  destroyed  by  the  far- 
ther flexion  of  the  tail  towards  the  left  side,  because  the 
principal  force  exerted  by  the  muscles  has  already  been  ex- 
pended in  the  motion  from  r  to  m,  in  hringing  it  to  a  straight 
line  with  the  body ;  and  the  force  which  carries  it  on  to  l  is 
much  weaker,  and,  therefore,  occasions  a  more  feeble  reac- 
tion. When  the  tail  has  arrived  at  the  position  l,  indicated 
by  the  dotted  outline,  a  similar  action  of  the  muscles  on  the 
right  side  will  create  a  resistance  and  an  impulse  in  the  di- 
rection of  K  L,  and  a  motion  of  the  whole  body  in  the  same 
direction,  c  a.  These  impulses  being  repeated  in  quick  suc- 
cession, the  fish  moves  forwards  in  the  diagonal  c  d,  interme- 
diate between  the  directions  of  the  two  forces.  By  bending 
the  whole  body  almost  in  a  circle,  and  then  suddenly  straight- 
ening it,  fishes  are  often  able  to  leap  to  the  top  of  a  high  ca- 
taract, in  ascending  against  the  stream  of  a  river. 

Such  being  the  plan  upon  which  progression  is  to  be  ef- 
fected, we  find  that  every  part  of  the  mechanism  of  the  fish 
is  calculated  to  promote  its  execution.  The  principal  mus- 
cular strength  is  bestowed  upon  the  movements  of  the  tail; 
and  the  largest  assemblage  of  muscles  consists  of  those  which 
give  it  the  lateral  flexions  that  have  been  just  described. 


288 


THE  MECHANICAL  FUNCTIONS. 


For  this  purpose,  all  the  important  viscera  are  placed  for- 
wards, and  crowded  towards  the  head.  No  room  is  allowed 
for  a  neck;  and  the  abdomen  may  be  almost  regarded  as 
continuous  with  the  head,  there  being,  properly,  no  inter- 
vening thorax;  for  the  respiratory  organs  are  situated  rather 
beneath  than  behind  the  head.  All  this  has  been  done  with 
a  view  to  leave  ample  scope  for  the  prolonged  expansion  of 
the  coccygeal  vertebrae,  and  of  their  muscles,  which  com- 
pose more  than  half, the  bulk  of  the  animal. 

Having  seen  how  all  impediments  to  the  free  motion  of 
the  tail  have  been  carefully  removed,  let  us  next  inquire  into 
the  mechanism  by  which  mobility  has  been  given  to  that 
organ.  The  first  peculiarity  we  meet  with  in  the  structure 
of  the  spine  of  fishes  is  the  mode  in  which  the  vertebras  are 
connected  together.  The  bodies  of  each  vertebra,  as  may 
be  seen  in  Figures  1S6  and  187,  are  hollowed  out,  both  be- 


fore and  behind,  (considering  the  spinal  column  as  extended 
horizontally,)  so  as  to  form  cup-like  hollows:  by  which 
means,  where  the  concave  surfaces  of  two  adjacent  vertebrae 
are  applied  to  one  another,  a  cavity,  having  the  shape  of  a 
double  cone,  is  formed  by  the  junction  of  the  margins  of 
these  conical  hollows.  These  cavities  are  distinctly  seen 
laid  open  in  Fig.  188,  which  represents  a  vertical  section  of 
three  adjacent  vertebrae  of  a  cod.  The  edges  that  are  in 
contact,  are  united  all  round  by  an  elastic  ligament,  which 
readily  yields  to  the  bending  of  the  vertebrae  upon  one  ano- 


SKELETON  OF  FISHES.  289 

ther  by  the  application  of  any  force  to  one  side  of  the  spine, 
and  restores  it  to  its  former  state,  when  the  force  has  ceased 
to  act.  The  extent  of  motion  in  each  joint  is  but  small;  but 
being  multiplied  in  the  whole  series,  the  resulting  effect  is 
considerable.  The  cavity  itself  is  filled  with  a  gelatinous, 
but  incompressible  fluid  substance,  which  constitutes  a  sphe- 
rical pivot  for  all  the  motions  of  the  joint. 

This  singular  kind  of  articulation  would  appear  framed 
with  a  view  to  allow  of  motion  in  all  directions.     Here, 
however,  the  motions  are  restricted  by  the  extension  of  the 
spinous  processes  (s,  s,  in  the  preceding  figures,)  which  in 
fishes  are  of  great  length;  so  that  they  effectually  prevent  all 
flexions  either  upwards  or  downwards,  and  limit  it  to  those 
from  side  to  side.   It  is  precisely  these  latter  kind  of  motions 
*.        that  are  wanted  in  the  fish,  for  striking  the  water  laterally, 
with  the  broad  vertical  surface  of  the  tail.    Processes  of  a  si- 
milar form  and  appearance,  f,  f,)  and  which  impede  any 
flexion  downwards,  are  generally  also  met  with  in  the  lower 
surface  of  the  spine,  and  more  especially  in  the  hinder  por- 
tion of  the  column.     These  are  the  i7ifenor  spinous  pro- 
cesses,  and,  like  the  superior,  they  also  form  an  arch,  through 
which  there  passes  the  continuation  of  the  abdominal  aorta, 
or  great  artery  which  proceeds  down  the  back.     The  num- 
ber of  vertebrae  is  very  various  in  different  fishes:  in  some 
they  are  multiplied  exceedingly,  as  in  the  shark,  where  there 
are  more  than  two  hundred. 

^  There  are  few  parts  of  the  structure  of  animals  that  ex- 
hibit more  remarkable  instances  of  the  law  of  gradation  than 
the  spine  of  fishes,  in  which  we  may  trace  a  regular  progress 
of  development  from  the  simplest  and  almost  rudimental 
condition  in  which  it  exists  in  the  M^xi?ie  and  the  Lam- 
prey,  to  that  of  the  most  perfect  of  the  osseous  tribes.     Its 
condition,  in  the  former  of  these  animals,  presents  a  close 
analogy  with  some  structures  that  are  met  with  in  the  mol- 
luscous, and  even  in  annulose  animals.    So  near  is  the  resem- 
blance  of  the  spinal  column  of  the  myxine,  more  especially, 
to  the  annular  condition  of  the  frame-work  of  the  vermes, 
Vol.  I.  37 


290  THE  MECHANICAL  FUNCTIONS. 

that  doubts  have  often  arisen  in  the  minds  of  naturalists 
whether  that  animal  ought  not  properly  to  be  ranked  among 
this  latter  class.     Its  pretensions  to  be  included  among  ihe 
vertebrata  are,  indeed,  but  slender  and  equivocal;   for,  in 
place  of  a  scries  of  bones  composing  the  vertebral  column, 
it  has  merely  a  soft  and  flexible  tube  of  a  homogeneous  and 
cartilaginous  substance,  exhibiting  scarcely  any  trace  of  divi- 
sion into  separate  rings,  but  appearing  as  if  it  were  formed 
of  a  continuous  hollow  cylinder  of  intervertebral  substance, 
usurping  the  place  of  the  vertebras,  which  it  is  the  usual  oflSce 
of  that  substance  to  connect  together,  and  having  in  its  axis 
a  continuous  canal  filled  with  gelatinous  fluid.     This,  how- 
ever, is  not  the  channel  intended  for  containing   the  spinal 
marrow,  for  that  nervous  cord  is  on  the  outside  of  this 
column.     The  cartilage,  indeed,  sends  out  no  processes  to 
bend  round  the  spinal  marrow,  and  forms  no  canal  for  its 
passage  and  protection.     The  nervous  matter  here  consists 
merely  of  two  slender  cords,  which  run  parallel  to  one  ano- 
ther in  a  groove  on  the  upper  part  of  the  spinal  column;  and 
these  cords  are  covered  only  by  a  thin   membrane,  the  pre- 
sence of  which  it  requires  very  minute  attention  to  detect. 
The   partial    protection    thus  afforded  to  so   important  an 
organ  is  not  greater  than  that  given  by  the  cartilaginous 
lamina  of  the  cuttle-fish,  which  in  form,  texture,  and  situa- 
tion, is  very  analogous  to  the  spine  of  the  myxine. 

As  we  ascend  from  this  rudimental  condition  of  the  spine, 
we  find  it,  in  the  lamprey,  more  distinctly  divided  into 
rounded  portions,  appearing  like  beads  strung  together. 
These  rudimental  bodies  of  vertebras  have  not  yet  completed 
the  cup-like  hollows  on  their  two  ends,  but  are  shaped  like 
rings,  being  perforated  in  the  centre,  so  as  still  to  form  a 
continuous  canal  throughout  the  whole  column. 

Proceeding  to  more  advanced  developments,  we  find,  in 

the  sturgeon  and  other  cartilaginous  fishes,  a  greater  conden- 

#  sation  of  substance  produced  by  the  deposition  of  granules 

of  osseous  matter;  the  central  canal  becomes  divided  into 

Jozenge-shaped  compartments  by  the  closing  in  of  the  sides 


STRUCTURE  OF  FISHES.  291 

of  the  body  of  each  vertebra.*  Frequently  the  sides  do  not 
quite  meet,  and  the  leaves,  which  are  developed  from  the 
upper  surfaces  of  the  vertebrae,  now  form  arches  over  the 
spinal  cord,  and  are  united  above  by  spinous  processes.  Yet 
the  whole  skeleton  in  these  fishes  remains  in  the  incipient 
stage  of  ossification,  being  more  or  less  cartilaginous;  and 
where  the  ossific  process  has  begun,  it  has  not  advanced  the 
length  of  producing  union  between  the  pieces  formed  from 
the  separate  centres  of  ossification.  Where  they  meet  with- 
out uniting,  they  form  no  sutures',  but  overlap  one  another. 
Thus  the  bony  structures  are  detached,  and  often  complete- 
ly isolated;  affording  to  the  physiologist  an  opportunity  of 
studying  the  earlier  stages  of  this  interesting  process,  and 
marking  with  distinctness  the  number  of  the  elements  of 
each  bone,  and  the  relative  situations  of  their  centres.  This 
knowledge  is  more  especially  of  importance  towards  under- 
standing the  formation  and  connexions  of  the  bones  of  the 
head,  which  are  very  numerous  and  complicated;  and  the 
investigation  of  which  has  been  prosecuted  with  extraordi- 
nary diligence  by  Geoflfroy  St.  Hilaire  and  other  continental 
zootomists. 

It  is  here,  more  especially,  that  we  obtain  the  clearest  evi- 
dence of  the  derivation  of  the  cranial  bones  from  vertebrae 
analogous  to  those  of  the  spine.  The  occipital  bone,  in  par- 
ticular, corresponds  to  a  spinal  vertebra  in  all  its  essential 
elements.  In  many  fishes,  the  body  of  this  bone,  being 
lengthened  out  to  form  the  posterior  part  of  the  basis  of  the 
skull,  becomes  the  basilar  portion.  We  find,  on  its  posterior 
surface,  the  same  cup-like  cavity  as  in  the  true  vertebrae, 
and  it  is  joined  to  the  next  vertebra  in  the  same  manner  as 
the  spinal  vertebrae  are  joined  to  each  other.     Its  crest  has 

•  A  small  aperture  still  remains,  establisliing-  a  communication  between 
the  cavities  the  whole  length  of  the  spine.  This  is  supposed  to  be  dcsig-ned 
to  obviate  the  compression  of  the  fluid  in  the  different  cells  or  cavities  during- 
the  motions  of  the  spine.  The  vertical  sections,  Fig.  189  and  190,  of  two 
contiguous  vertebra  in  different  fishes,  will  convey  an  idea  of  this  gradation 
of  development. 


292  THE  MECHANICAL  FUNCTIONS. 

the  exact  shape  of  a  spinous  process.  In  front  the  basilar 
bone  is  united  to  the  sphenoid  bone,  which,  with  the  vaulted 
roof  that  springs  from  the  sides  of  both  these  bones,  like  the 
leaves  and  spinous  processes  of  the  vertebrse,  form  together 
a  long  cranial  cavity.  This  cavity  is  placed  in  a  direct  line 
with  the  spinal  canal,  and  contains  the  nervous  tubercles 
which  constitute  the  brain.  Yet  the  brain  does  not  com- 
pletely fill  this  cavity;  for  a  space  is  still  left,  which  is  occu- 
pied by  a  pulpy  substance.  In  like  manner,  the  accordance 
of  the  other  cranial  bones  with  vertebrae,  has  been  attempted 
to  be  traced;  but  in  proportion  as  we  recede  from  the  cen- 
tral parts  of  the  spine,  this  correspondence  is  less  distinct,  in 
consequence  of  the  various  degrees  of  development  which 
these  several  elements  have  received,  in  order  to  adapt  them 
to  particular  purposes  relating  to  sensation,  to  the  prehen- 
sion and  deglutition  of  the  food,  and  also  to  aquatic  respira- 
tion. It  is  impossible,  however,  without  exceeding  the  li- 
mits within  which  I  must  here  confine  myself,  to  enter  into 
the  details  of  structure  which  would  be  requisite  in  order  to 
render  this  subject  sufficiently  intelligible. 

The  rest'of  the  skeleton  of  fishes  is  extremely  simple.  In 
many,  as  in  the  Ray  and  Tetrodon,  there  are  no  ribs. 
Where  these  bones  exist,  they  are  articulated  with  the  ex- 
tremities of  the*  transverse  processes  of  the  vertebrse,  of 
which  they  appear  to  be  merely  continuations,  or  appendi- 
ces. There  is  generally  no  sternum  to  which  they  can  be 
attached  below:  in  a  few  fishes  only,  such  as  i\iQ  herring 
and  the  dory,  we  find  rudiments  of  this  bone,  consisting  of 
a  few  pieces  placed  in  a  line  on  the  lower  part  of  the  trunk.^ 

The  parts  of  the  skeleton  of  fishes,  which  correspond  to 
the  arms  and  legs  of  quadrupeds,  are  the  pectoral  and  ven- 
tral fins  (marked  respectively  by  the  letters  p  and  v  in  Fig. 
184.)     The  former  are  met  with,  with  but  few  exceptions, 

•  The  bony  arches  arising  from  the  skull,  which  support  the  bronchiae,  or 
gills,  have  been  considered  as  the  bones  corresponding  to  the  ribs  of  terres- 
trial quadrupeds;  and  if  this  view  were  taken  of  them,  it  would  tend  to  con- 
firm the  analogy  of  the  cranial  bones  to  the  spinal  vertebra. 


STRUCTURE  OF  FISHES. 


293 


in  all  fishes;  and  they  consist  of  a  scries  of  osseous  pieces, 
in  which  we  may  often  recognise  with  tolerable  precision 
the  analogous  bones  composing  the  anterior  extremities  of 
a  quadruped;  such  as  the  scapula,  clavicle,  humerus,  ulna, 
and  radius.*'  These  two  latter  bones  are  very  distinctly 
marked  in  the  Lophhts  jmcatorius,  or  Ajigler,  as  may  be 
seen  in  Fig.  191,  where  b  is  the  scapula;  c,  the  clavicle;  u, 
the  ulna;  and  r,  the  radius.     The  carpus  may  also  be  recog- 


nised in  a  chain  of  small  bones,  w^,  interposed  between  the 
radius  and  the  Phalanges,  z.  In  the  Ray  these  phalanges 
are  very  numerous,  and  each  is  divided  into  several  pieces 
by  regular  articulations:  these  are  shown  in  Fig.  192:  they 
are  arranged  close  to  one  another  in  one  plane,  and  form  an 
effectual  base  of  support  to  the  integument  which  covers 
them.  The  scapula,  according  to  Cuvier,  is  sometimes  de- 
tached from  the  rest  of  the  skeleton,  and  at  other  times  con- 
nected with  the  spine:  in  most  cases,  however,  it  is  sus- 
pended from  the  cranium;  a  fact  which  may  be  cited  in 

•  Those  anatomists  who  are  fond  of  pursuing"  the  theory  of  analogies, 
maintain  that  all  these  bones  are  merely  developments  of  certain  ribs,  pro- 
ceeding- from  the  spine  in  its  anterior  parts.  A  similar  orig-in  has  been  as- 
signed to  the  pieces  of  bone  to  which  the  ventral  fins  are  attached:  but  it  is 
difficult  to  reconcile  this  tlieory  with  the  fact  that  these  bones  do  not  pro- 
ceed from  the  spine,  and  are  quite  detached  from  the  rest  of  the  skeleton. 
It  is  evident,  therefore,  that  if  they  are  to  be  considered  as  analog-ous  to  the 
bones  of  the  hinder  extremities  in  the  mammalia,  they  arc  in  a  condition  of 
very  imperfect  development. 


294 


THE  MECHANICAL  FUNCTIONS. 


farther  corroboration  of  the  analogy  which  the  cranial  bones 
have  to  vertebrae. 

In  the  ray  and  the  shark  tribes,  both  the  anterior  and  pos- 
terior extremities  are  sup- 
ported by  arches  of  bones, 
formino;  a  sort  of  belt.  This 
structure  is  an  approach  to 
that  which  obtains  in  many 
reptiles,  and  indicates  a  farther  step  in  the  regular  progress 
of  development.  This  belt  in  the  ray  is  shown  in  Fig.  193. 
In  examining  that  part  of  the  skeleton  of  fishes  which 
corresponds  to  the  posterior  extremity,  we  observe  the  total 
absence  of  both  femur  and  tibia;  but  the  bones  of  the  toes 
are  attached  to  a  set  of  small  bones,  which  appear  to  act  the 
part  of  a  pelvis,  but  which,  in  consequence  of  their  not  being 
connected  with  the  spine,  have  no  determinate  situation,  and 
are  found  at  various  distances  from  the  head  in  different 
fishes.  They  appear  emancipated  from  the  restraints  to 
which  they  would  have  been  subjected  had  they  been  fixed 
to  a  sacrum,  or  to  any  particular  part  of  the  spine:  and  we 
find  them,  accordingly,  often  placed  considerably  forwards; 
and  in  some  instances,  as  in  the  Subhrachieni,  even  anteri- 
orly to  the  pectoral  fins,  which  are  the  true  arms  of  the  ani- 
mal. But  in  one  whole  order  of  fishes,  the  ^djjodes,  there 
is  not  even  a  vestige  of  ventral  fins,  nor  are  any  pelvic  bones 
provided  for  their  support.  This  is  the  case  with  the  Eel, 
the  Gymnotus,  &c.  In  a  few  species  there  is  also  a  total 
absence  of  pectoral  as  well  as  ventral  fins. 

The  dorsal  fins  are  supported  by  a  series  of  slender  bones 
(d,  Fig.  1S4,)  which  are  joined  to  the  spinous  processes  of 
the  vertebras,  and  are  formed  from  distinct  centres  of  ossi- 
fication. These  rays,  as  they  are  called,  are  sometimes 
destined  to  grow  to  so  considerable  a  length,  as  to  require 
being  subdivided  into  many  pieces,  in  order  to  lessen  the 
danger  of  fracture,  to  which  a  very  long  filajiient  of  bone 
would  have  been  exposed,  and  also  to  allow  of  a  greater  de- 
gree of  flexibility.     These  rays  assume  branched  forms  from 


MUSCULAR  SYSTEM  OP  FISHES. 


295 


the  farther  subdivision  of  their  parts,  and  when,  for  the  pur- 
pose of  adding  strength  to  the  fin,  it  becomes  necessary  to 
multiply  the  points  of  support,  intermediate  bones  are  de- 
veloped, serving  as  the  basis  of  the  rays.  Convenience  re- 
quires that  they  should  be  detached  from  the  ends  of  the 
spinous  processes,  which  is  their  usual  position,  and  placed 
between  them:  wdieh  in  this  situation,  they  bear  the  name 
of  interspinoiis  bones;  and  when  a  still  greater  length  of 
osseous  support  is  wanted,  new  centres  of  ossification  are 
developed  at  their  extremities,  giving  rise  to  a  series  of  ad- 
ditional pieces,  joined  end  to  end,  and  carrying  out  the  in- 
terspinous  bone,  and  the  ray  which  terminates  it,  to  a  con- 
siderable distance.  This  structure  is  distinctly  seen  in  the 
small  dorsal  fins  of  the  Mackeral.  The  anal  fins,  which 
are  situated  on  the  lower  side  of  the  body,  in  the  vertical 
plane,  and  next  to  the  tail,  are,  in  like  manner,  supported 
by  rays,  having  the  same  parallel,  or  fan-like  arrangement 
as  the  preceding.  The  caudal  fin,  or  terminal  expansion  of 
the  tail  has  also  a  similar  structure. 

The  muscles  of  fishes  compose  a  large  portion  of  the  bulk 
of  the  body,  but  they  are  arranged  in  a  less  complex  man- 
ner than  those  of  the  animals  of  the  higher  classes.  Those 
which  appear  immediately  underneath  the  integuments  are 
shown  in  Fig.  194,  where  m,  m  are  the  great  lateral  muscles, 


producing  the  flexion  of  the  body  and  tail:  d  is  the  dorsal  fin, 
which  is  raised  by  the  muscle  d;  the  pectoral  fin,  expanded 
by  the  muscle  p:  v,  the  ventral  fin,  moved  by  the  mu.scles 
situated  at  v:  a,  the  anal  fin,  in  like  manner  moved  by  nms- 
cles  at  its  base  a:  and  c,  the  caudal  fin,  the  muscles  for 
moving  which  arc  seen  at  c:  o  is  the  operculum,  or  flap. 


296  THE  MECHANICAL  FUNCTIONS. 

which  covers  the  gills :  and  n,  the  nasal  cavities,  or  organs 
of  smell.  The  form  of  the  hody,  and  disposition  of  the  skele- 
ton, allow  of  their  being  inserted  immediately  on  the  parts 
which  they  are  intended  to  approximate.  Hence  the  use  of 
long  tendinous  chords  is  dispensed  with.^' 

The  actions  of  the  muscles  are  easily  understood  from  the 
nature  of  their  insertions.  In  general,  the  direction  of  the 
fibres  is,  in  some  degree,  oblique,  with  reference  to  the  mo- 
tion performed.  Two  series  of  muscles  are  provided  for  the 
movements  of  the  tail,  which  consist  almost  exclusively  of 
lateral  flexion,  the  whole  spine  in  some  degree  participating 
in  this  motion.  These  muscles  occupy  the  upper  and  lower 
portions  of  the  trunk :  their  limits  being  strongly  marked  by 
a  line  running  longitudinally  the  whole  length  of  the  body  on 
each  side.  The  inchnation  of  their  fibres  is  somewhat  diffe- 
rent in  each.  The  advantage  in  point  of  velocity  of  action 
which  results  from  this  obhquity  has  already  been  pointed 
out. 

Those  fins  which  are  in  pairs  are  capable  of  four  motions ; 
namely,  those  of  flexion  and  extension,  and  also  those  of  ex- 
panding and  closing  the  rays ;  for  each  of  which  motions  ap- 
propriate muscles  are  provided :  and,  indeed,  each  ray  is  fur- 
nished with  a  distinct  muscular  apparatus  for  its  separate  mo- 
tion ;  and  these  smaller  muscles  regulate  with  great  nicety  all 
the  movements  of  the  fins,  expanding  or  closing  them  like  a 
fan,  according  as  their  action  is  to  be  strengthened  or  relaxed. 
This  feathering  of  the  fin,  as  it  may  be  called,  takes  place  in 
most  fishes,  and  is  particularly  observable  in  the  tail  of  the 
Esox,  or  pike  tribe.  Each  ray  of  these  fins,  indeed,  is  fur- 
nished with  a  distinct  muscular  apparatus,  for  its  separate 
motion. 

Whatever  analogy  may  exist  in  the  structure  of  the  fins 

*  Between  the  layers  of  flesh,  however,  there  occur  slender  semi-transpa- 
rent tendons,  which  give  attachment  to  a  series  of  short  muscular  fibres  pass- 
ing nearly  at  right  angles  between  the  surfaces  of  the  adjoining  plates. 
See  Sir  A.  Carlisle's  account  of  this  structure  in  the  Philosophical  Transac- 
tions for  1806. 


SWIMMING  BLADDER  OF  FISHES.  297 

of  fishes  and  the  feet  of  quadrupeds,  there  is  none  in  the 
manner  in  which  they  are  instrumental  in  clFccting  pro- 
gressive motion.  Tlie  great  agent  h}^  which  the  fish  is  im- 
pelled forwards  is  the  tail:  the  fins,  wliich  correspond  to  the 
extremities  of  land  animals,  are  useful  chiefly  for  the  pur- 
poses of  turning,  stopping,  or  inclining  the  hody,  and  for 
retaining  it  in  its  proper  position.  The  single  fins,  or  those 
which  are  situated  in  a  vertical  plane,  passing  through  the 
axis  of  the  hody,  (the  mesial  plane,)  prevent  the  rolling  of 
the  body,  wdiile  the  fish  darts  forwards  in  its  course.  The 
fins  that  are  in  pairs  (that  is  the  pectoral  and  the  ventral 
fins,)  by  their  alternate  flexions  and  extensions,  act  like 
oars;  while  they  are  capable,  at  the  same  time,  of  expanding 
and  of  closing  the  rays,  like  the  opening  and  shutting  of  a 
fan,  according  as  their  action  is  required  to  be  eflective,  or 
the  contrary.  All  these  auxiliary  instruments  arc  chiefly 
serviceable  in  modifying  the  direction,  and  adjusting  the 
variations  of  force  derived  from  the  impulse  of  the  tail. 
:^They  are  employed,  also,  in  suddenly  checking  or  stopping 
the  motion,  and  giving  it  a  more  rapid  acceleration.  But 
still  the  tail  is  the  most  powerful  of  the  instruments  for  pro- 
gression, being  at  once  a  vigorous  oar,  an  accurate  rudder, 
and  a  formidable  weapon  of  ofience. 

Independently  of  these  external  instruments  of  progres- 
sion, most  fishes  are  provided  with  internal  means  of  changing 
their  situation  in  the  water.  The  structure  by  which  this 
efiect  is  accomplished  is  one  of  the  most  remarkable  in- 
stances that  is  met  with  of  an  express  contrivance  for  a  spe- 
cific purpose,  and  of  the  employment  of  an  agency  of  a  class 
different  from  that  of  the  mechanical  powers  usually  resorted 
to  for  effecting  the  same  object.  We  have  seen  that  if  the 
body  of  a  fish  were  heavier  than  an  equal  bulk  of  water,  and 
if  no  muscular  exertions  were  made,  it  must  necessarily  de- 
scend in  that  fluid.  If,  on  the  contrary,  it  were  specifically 
lighter,  it  would  as  necessarily  rise  to  the  surface.  Were 
the  animal  to  acquire  the  power  of  altering,  at  pleasure,  its 
specific  gravity,  it  would  then  possess  the  means  of  rising 
Vol.  I.  38 


298 


THE  MECHANICAL  FUNCTIONS. 


or  sinking,  without  calling  into  action  either  the  fins  or  the 
tail.  Such  is  precisely  the  object  of  a  peculiar  mechanism, 
which  nature  has  provided  in  the  interior  of  the  body  of  the 
fish.  A  large  bladder,  filled  with  air,  has  been  placed  im- 
mediately under  the  spine,  in  the  middle  of  the  back,  and 
above  the  centre  of  gravity.  This  is  known  by  the  name 
of  the  air-bladder,  or  the  swhnming  bladder,  and  in  the 
cod-fish  it  is  called  the  sound.  It  frequently,  as  in  the 
Carp,  consists  of  two  bladders  (a,  b.  Fig.  195)  joined  end- 


wise, and  communicating  with  each  other  by  a  narrow  neck.* 
When  distended  with  air,  it  renders  the  whole  fish  specifi- 
cally lighter  than  the  surrounding  water;  and  the  fish  is 
thus  buoyed  up,  and  remains  at  the  surface  without  any  ef- 
fort of  its  own.  On  compressing  the  bladder,  by  the  action 
of  the  surrounding  muscles,  the  included  air  is  condensed, 
the  specific  gravity  of  the  whole  body  is  increased,  and  the 
fish  sinks  to  the  bottom.  On  relaxing  the  same  muscles, 
the  air  recovers  its  former  dimensions,  and  the  fish  is  again 
rendered  buoyant.  Can  there  be  stronger  evidence  of  de- 
sign than  the  placing  of  this  hydrostatic  apparatus,  acting 
upon  philosophical  principles,  in  the  interior  of  the  organi- 
zation, for  a  purpose  so  definite  and  unequivocal? 

In  several  tribes  of  fishes  there  is  a  canal  (c  d)  establish- 
ing a  communication  between  this  bladder  and  the  stomach, 
or  the  gullet  (o;)  so  that  by  compressing  the  bladder,  a  quan- 
tity of  air  may  be  forced  out,  and  a  very  sudden  increase  of 

*  There  is  great  variety  in  the  form  and  structure  of  tlie  air-bladder  in 
different  fishes.  Sometimes  it  contains  a  large  glandular  body  of  a  peculiar 
structure,  which  has  been  conjectured  to  be  an  apparatus  for  secreting  air 
from  the  blood:  but  it  is  by  no  means  very  generally  met  with. 


SWIMMING  BLADDER  OP  FISHES.  29.Q 

specific  gravity  produced;  followed,  of  course,  by  a  quick 
descent.     When,  by  any  accident,  the  air  bladder  has  been 
opened,  or  has  burst,  so  that  all  the  air  has  escaped,  the  fish 
is  seen  to  grovel  at  the  bottom,  lying  on  its  back,  and  can 
never  afterwards  rise  to  the  surface.     On  the  other  hand,  it 
occasionally  happens  that  a  fish  which  has  remained  too  long 
at  the  surface  of  the  sea,  exposed  to  the  scorching  rays  of  a 
tropical  sun,  suddenly  finds  itself  retained  against  its  will  at 
the  surface,  because  the  bladder  has  beconie  over  distended 
by  the  heat,  and  resists  all  the  efforts  which  the  animal  can 
make  to  compress  it.     It  thus  continues  floating,  until  the 
coolness  of  the  night  has  again  condensed  the  air  in  the  blad- 
der to  its  former  bulk,  and  restored  the  power  of  descending. 
Some  tribes  of  fish  are  totally  unprovided  with  an  air- 
bladder.     This  is  the  case  with  the  flounder,  the  sole,  and 
other  genera  of  a  flat  shape,  forming  the  family  of  Pleuron- 
cetes.     They  are  chiefly  inhabitants  of  sand-banks,  or  other 
situations  where  they  are  comparatively  stationary,  seldom 
moving  to  a  distance,   or  rising  much  in  the  water;  and 
when  they  do  so,  it  is  with  manifest  eflfort,  for  their  ascent 
must  be  accomplished  entirely  by  the  continued  beating  and 
flapping  of  the  water  with  their  expanded  pectoral  fins.     It 
is  only  the  larger  fish  of  this  form,  such  as  rays,  which  have 
very  voluminous  and  powerful  pectoral  fins  for  striking  the 
water  downwards  with  considerable  force,  that  can  rise  with 
facility  without  the  assistance  of  an  air-bladder.     In  these, 
the  lateral  fins,  which  are  enormous  expansions  of  the  pec- 
toral fins,  may  be  compared  to  wings,  their  vertical  action 
on  the  water  being  similar  in  eflfect  to  the  corresponding 
movements  of  a  bird,  when  it  rises  vertically  in  the  air. 
Those  fishes  which  swim  rapidly,  and  frequently  ascend  and 
descend  in  the  water,  are,  in  general,  provided  with   the 
largest  air-bladders. 

In  studying  the  varieties  presented  by  the  forms  of  the 
fins  in  different  tribes  of  fishes,  we  find  the  same  constant 
relation  preserved  with  the  particular  situations  and  circum- 
stances in  which  they  are  placed.     The  dorsal  fins,  which 


^'V 


300  THE  MECHANICAL  TUNCTIONS. 

are  more  especially  useful  for  steadying  the  body,  arc  long- 
est in  those  fishes  which  inhabit  the  most  stormy  seas.  The 
most  voracious  tribes,  which  incessantly  pursue  their  prey, 
are  furnished  with  most  pow^erful  muscles,  and  possess  the 
greatest  means  of  rapid  progression.  On  the  other  hand, 
many  of  the  more  pacific,  and  weaker  species  are  studiously 
guarded  by  a  dense  and  hard  integument,  serving  as  a  shield 
against  the  attacks  of  enemies,  and  often  armed  v^ith  sharp 
points,  which  are  sufficient  to  repel  the  most  daring  assail- 
ant. The  Balisies  is  covered  with  scales  of  singular  hard- 
ness, closely  set  together,  and  frequently  having  rough 
edo-es.  The  Ostrucioii,  or  trunk  fish,  instead  of  these  scales, 
is  provided  wnth  a  kind  of  coat  of  mail,  composed  of  osseous 
plates,  curiously  joined  together,  like  a  tesselated  pavement, 
and  reminding  us  of  the  arrangements  we  have  seen  adopted 
in  the  calcareous  coverings  of  the  echinida. 

Some  of  the  cartilaginous  fishes  are,  in  like  manner,  pro- 
tected by  calcareous  plates,  appended  to  the  integuments. 
There  is  a  row  of  plates  of  this  kind,  of  a  quadrangular 
shape,  which  passes  along  the  middle  of  the  back  in  the  stur- 
geon: and  the  whole  body  of  the  Ostracion,  or  Trunk-fish, 
is  covered  with  osseous  scales.  All  these  have  no  imme- 
diate relation  to  the  skeleton,  but  are  apparently  remnants 
of  inferior  types,  of  which  one  of  the  prevailing  characters 
is  the  external  situation  of  the  protecting  organs. 

Diodons  and  Tetrodons  are  remarkable  for  being  provided 
with  the  means  of  suddenly  assuming  a  globular  form,  by 
swallowing  air,  which,  passing  into  the  crop  or  first  stomach, 
blows  up  the  whole  animal  like  a  balloon.  The  abdominal 
renion  beino;  thus  rendered  the  lightest,  the  body  turns  over, 
the  stomach  becoming  the  uppermost  part;  and  the  fish  floats 
upon  its  back,  without  having  the  power  of  directing  itself 
durino:  this  state  of  forced  distention.  But  it  is  while  lying 
thus  bloated  and  passive,  at  the  mercy  of  the  waves,  that  this 
animal  is  really  most  secure;  for  the  numerous  spines,  with 
which  the  surface  of  the  body  is  universally  beset,  are  raised 
and  erected  by  the  stretching  out  of  the  skin,  thus  present- 


MOVEMENT  OF  FISHES.  301 

mg  an  armed  front  to  the  enemy,  on  whatever  side  he  may 
venture  to  begin  the  attack. 

There  is  a  numerous  family  of  fishes,  found  in  tlie  seas  of 
India,  so  constructed  as  to  be  able  to  crawl  on  land  to  some 
distance  from  the  shore.  One  of  these,  the  Ferca  scandens, 
is  even  capable  of  climbing  on  the  trees  which  grow  on  the 
coast* 

If  we  consider  the  density  of  the  medium  which  fishes 
have  to  traverse,  the  velocity  with  which  they  move  will 
appear  surprising.  They  dart  through  the  water  with  ap- 
parently as  much  ease  and  rapidity  as  a  bird  flics  through 
the  air.  Although  this  may  partly  be  accounted  for  by  the 
size  of  their  muscles,  and  the  advantageous  mode  of  their  in- 
sertion, yet  these  advantages  would  avail  but  little,  were  it 
not  for  the  sudden  manner  in  which  their  power  is  exerted. 
Where  the  great  length  and  flexibility  of  the  spine  tend  to 
impair  the  force  with  which  the  tail  strikes  the  water,  the 
resulting  motion  is  slow  and  desultory,  as  is  the  case  with 
eels,  and  other  fishes  of  the  same  elongated  construction.! 
Most  fishes,  however,  move  with  the  utmost  rapidity,  and 
with  scarcely  any  visible  effort;  and  perform  long  journeys 
without  apparent  fatigue.  The  Salmon  has  been  known  to 
travel  at  the  rate  of  sixteen  miles  an  hour  for  many  days  to- 
gether. Sharks  often  follow  ships  across  the  Atlantic,  not 
only  outstripping  them  in  their  swiftest  sailing,  but  playing 
round  them  on  every  side,  just  as  if  the  vessel  were  at  rest. 

*  See  the  account  given  by  Lieutenant  Daldorff;  Linnean  Transactions,  III. 
62.  I  shall  have  occasion  to  notice,  in  the  sequel,  the  remarkable  conforma- 
tion of  the  respiratory  organs  of  these  and  other  fishes,  which  enable  them  to 
live,  for  a  time,  out  of  their  natural  element. 

■J- Carlisle,  Phil.  Trans,  for  1806,  p.  9. 


(     302     ) 


CHAPTER  VIII. 


REPTILIA. 


§  1.    Terrestrial  Vertehrata  in  general. 

The  numerous  tribes  of  vertebrated  animals  wbich  are 
Strictly  terrestrial,  or  destined  to  move  on  land,  differ  widely 
in  their  modes  of  progression,  and  in  the  mechanical  advan- 
tages of  their  formation.  The  greater  number  are  quadru- 
peds; some  formed  for  climbing  trees,  others,  for  burrowing 
in  the  earth;  some  for  treading  on  sandy  plains,  some  for 
scaling  precipices.  A. few  seem  scarcely  capable  of  ad- 
vancing; others  outstrip  the  winds  in  fleetness.  Some  fami- 
lies of  reptiles  are  entirely  destitute  of  any  external  organs 
of  motion,  the  whole  trunk  of  the  body  resting  on  the  ground : 
while  man  occupies  a  place  where  he  stands,  alone,  being 
distinguished  by  the  exclusive  faculty  of  permanently  sus- 
taining himself  on  the  lower  extremities. 

In  reviewing  the  developments  and  the  mechanical  func- 
tions exhibited  by  so  great  a  diversity  of  structures,  I  shall 
commence  with  an  examination  of  those  amphibious  reptiles 
which  appear  to  form  an  intermediate  link  in  the  chain  con- 
necting the  strictly  aquatic,  with  the  terrestrial  vertebrated 
animals:  then,  taking  up  this  latter  series,  I  shall  consider 
the  more  sim.ple  conformation,  and  less  perfect  motions  of 
terrestrial  animals  destitute  of  limbs;  and  gradually  ascend 
to  those  in  which  the  support  and  progression  of  the  body  is 
effected  by  extremities,  more  and  more  artificially  formed: 
concluding  w^ith  the  human  structure,  which  terminates  this 
extensive  series. 


BATRACHIA. 


303 


§  2.  Balrachia. 


The  order  of  Balrachia,  or  Amphibious  Reptiles,  con- 
stitutes the  first  step  in  the  transition  from  aquatic  to  terres- 
trial vertebrata.  It  is  more  particularly  the  function  of  res- 
piration that  requires  to  be  modified,  in  consequence  of  the 
change  of  element  in  which  the  animal  is  to  reside;  and  as 
if  it  had  been  necessary,  conformably  to  the  laws  of  animal 
creation,  that  this  change  should  not  be  abruptly  made,  we 
find  that  Batrachian  reptiles,  with  which  this  series  com- 
mences, are  constructed,  at  first,  on  the  model  of  fishes; 
breathing  the  atmospheric  air  contained  in  the  water  by 
means  of  gills,  and  moving  through  the  fluid  by  the  same 
instruments  of  progression  as  fishes,  which,  indeed,  they  ex- 
actly resemble  in  every  part  of  their  mechanical  conforma- 
tion. The  tadpole,  which  is  the  young  of  the  frog,  is,  at 
first,  not  distinguishable  in  any  circumstance  of  its  internal 
skeleton,  or  in  the  disposition  of  its  vital  organs  from  the 
class  of  fishes.     The  head,  indeed,  is  enlarged,  but  the  body 

19; 


immediately  tapers  to  form  a  lengthened  tail,  by  the  pro- 
longation of  the  spinal  column,  which  presents  a  numerous 
series  of  coccygeal  vertebrae,  furnished  with  a  vertical  ex- 
pansion of  membrane  to  serve  as  a  caudal  fin,  and  with  ap- 
propriate muscles  for  executing  all  the  motions  required  in  # 
swimming.  The  appearance  of  the  tadpole,  in  its  early 
stage  of  development,  is  seen  in    Fig.   197  and  198,  the 


304  THE  MECHANICAL  FUNCTIONS. 

former  being  a  side,  and  the  latter  an  upper  view  of  that 
animal. 

Yet,  with  all  this  apparent  conformity  to  the  structure  of 
a  strictly  aquatic  animal,  the  tadpole  contains  within  its 
organization  the  germs  of  a  higher  development.  Prepara- 
tions are  silently  making  for  a  change  of  habitation,  for  the 
animal's  emerging  from  the  waters,  for  the  reception  of  at- 
mospheric air  into  new  cavities,  for  the  acquisition  of  limbs 
suited  to  new  modes  of  progression;  in  a  word^  for  a  terres- 
trial life,  and  for  all  the  attributes  and  powers  which  belong 
to  quadrupeds.  The  succession  of  forms,  which  these  meta- 
morphoses present,  are  in  themselves  exceedingly  curious, 
and  bear  a  remarkable  analogy  with  the  progress  of  the  trans- 
formations of  those  insects,  which  in  the  first  stages  of  their 
existence  are  aquatic.  To  the  philosophic  inquirer  into  the 
marvellous  plans  of  creation,  the  series  of  changes  which 
mark  these  singular  transitions  cannot  fail  to  be  deeply  in- 
teresting; and  occurring  as  we  here  find  them,  among  a  tribe 
of  animals  allied  to  the  more  perfect  forms  of  organization, 
they  afibrd  us  a  better  opportunity  of  exploring  the  secrets 
of  their  development  by  tracing  them  from  the  earlier  stages 
of  this  complicated  process  so  full  of  mystery  and  of  won- 
der. 

The  egg  of  the  frog  (Fig.  196)  is  a  round  mass  of  trans- 
parent nutritive  jelly,  in  the  centre  of  which  appears  a  small 
black  globule.  By  degrees  this  shapeless  globule  exhibits 
the  appearance  of  a  head  and  tail,  and  in  this  form  it  emerges 
from  its  prison,  and  moves  briskly  in  the  water.  From 
the  sides  of  the  neck  there  grow  out  feathery  tufts,  (Fig. 
198,  B,  B,)  which  float  loosely,  and  without  protection,  in 
the  surrounding  fluid.  These,  however,  are  mere  tempo- 
rar}^  organs,  for  they  serve  the  purposes  of  respiration  only 
until  the  proper  gills  are  formed,  and  they  then  shrink  and 
become  obliterated.  The  true  gills,  or  branchise,  are  con- 
tained within  the  body,  and  are  four  in  number  on  each 
side,  constructed  on  a  plan  very  similar  to  those  of  fishes. 
Retaining  this  aquatic  constitution,  the  tadpole  rapidly  in- 


DEVELOPMENT  OF  THE  BATRACHIA.  305 

creases  in  size  and  in  activity  for  several  weeks.  In  the  mean 
time  the  legs,  of  which  no  trace  was  at  first  apparent,  have 
commenced  tlicir  o;rowlh.  The  hind  ]e2;s  are  the  first  to 
make  their  appearance,  showing  their  emhryo  forms  with- 
in the  transparent  coverings  of  the  hinder  part  of  the 
trunk,  just  at  the  origin  of  the  tail.  These  are  soon  suc- 
ceeded by  the  four  legs,  which  exactly  follow  the  hind 
legs,  in  all  the  stages  of  their  development,  until  they 
have  acquired  their  due  proportion  to  the  size  of  the 
trunk.  The  animal  at  this  period  wears  a  very  ambiguous 
appearance,  partaking  of  the  forms  both  of  the  frog  and  of 
the  lizard,  and  swimming  both  by  the  inflections  of  the  tail, 
and  the  irregular  impulses  given  by  the  feet.  This  inter- 
val is  also  employed  by  this  amphibious  being,  in  acquiring 
the  faculty  of  respiring  atmospheric  air.  We  observe  it 
rising  every  now  and  then  to  the  surface,  and  cultivating  its 
acquaintance  with  that  element,  into  which  it  is  soon  to  be 
raised;  occasionally  taking  in  a  mouthful  of  air,  which  is  re- 
ceived into  its  newly  developed  lungs,  and  afterwards  dis- 
charging it  in  the  form  of  a  small  bubble.  AVhen  the  ne- 
cessary internal  changes  are  at  length  completed,  prepara- 
tions are  made  for  getting  rid  of  the  tail,  which  is  now  a 
useless  member,  and  which,  ceasing  to  be  nourished,  dimi- 
nishes by  degrees  leaving  only  a  short  stump,  which  is  soon 
removed.  The  gills  are  by  this  time  shrunk,  and  rapidly 
disappear,  their  function  being  superseded  hy  the  lungs, 
which  have  been  called  into  play;  and  the  animal  now 
emerges  from  the  water,  and  begins  a  new  mode  of  existence, 
having  become  a  perfect  frog,  (Fig.  199.)  It  still,  however, 
retains  it  aquatic  habits,  and  swims  with  great  ease  in  the 
water  by  means  of  its  hind  feet,  which  are  very  long  and 
muscular,  and  of  which  the  toes  are  furnished  with  a  broad 
web,  derived  from  a  thin  extension  of  the  integuments. 

No  less  curious  are  the  changes  which  take  place  in  all  the 
other  organs  for  the  purpose  of  effecting  the  transformations 
rendered  necessary  by  this  entire  alteration  in  all  the  ex- 
ternal circumstances  of  that  animal, — this   total  reversal  of 

Vol.  I.  ,39 


306 


THE  MECHANICAL  FUNCTIONS. 


its  wants,  of  its  habits,  of  its  functions,  and  of  its  very  con- 
stitution. I  shall  have  occasion  to  notice  several  of  these 
transitions  when  reviewing  the  other  functions  of  the  animal 
economy:  but  at  present  our  concern  is  chiefly  with  the 
structure  of  the  frame  in  its  mechanical  relations  to  progres- 
sive motion.  In  order  to  form  a  correct  idea  of  these  re- 
lations, it  will  be  necessary  to  notice  the  leading  peculiari- 
ties of  the  skeletons  of  this  tribe  of  animals. 

The  skeleton  of  the  adult  frog  is  shown  in  Fig.  200;  from 
which  it  will  be  seen  that  the  spinal  column  is  comparatively 


much  shorter  than  that  of  fishes,  or,  indeed,  of  any  other 
class  of  animals;  for  it  consists  of  only  eight  vertebrae,  ex- 
clusive of  those  which  have  united  to  form  the  os  coccygis. 
It  was  evidently  the  intention  of  nature  to  consolidate  the 
frame-work  of  the  trunk,  in  which  flexibility  was  not  re- 
quired for  progressive  motion:  the  performance  of  that  func- 
tion being  transferred  to  the  hind  extremities,  which  are  ex- 
ceedingly large  in  proportion  to  the  rest  of  the  body.  There 
is  a  tendency  in  every  part  of  the  skeleton  to  develope  itself 
in  a  transverse  direction,  while  the  trunk  is  shortened  as 
much  as  possible. 

The  mode  in  which  the  vertebras  are  articulated  together, 


SKELETON  OF  THE  BATRACHIA.  307 

differs  widely  from  what  we  have  seen  in  fishes,  and  ap- 
proaches to  the  structure  of  tlie  higher  chisses  of  vertel)rata. 
The  body  of  each  vertebra,  instead  of  having  at  its  posterior 
surface  a  cup-like  cavity,  terminates  by  a  projecting  ball, 
which  is  received  into  the  cavity  in  the  anterior  surface  of 
the  next  vertebra,  so  as  to  compose  a  true  ball  and  socket 
joint,  capable,  when  other  circumstances  permit,  of  a  rotato- 
ry motion.     But  the  vertebrae  of  the  tadpole,  as  we  have 
seen,  are  constructed  on  the  model  of  those  of  a  fish;  that  is, 
have  cup-like  cavities  on  both  their  surfaces,  which  play  on 
balls  of  soft  elastic  matter,  interposed  between  them.     We 
should  naturally  be  curious  to  learn  the  mode  in  which  the 
transition  from  this  structure  to  that  of  the  frog  is  accom- 
plished.    By  carefully  watching  the  progress  of  ossification, 
while  this  change  is  taking  place,  Dutrochet  found  that  the 
gelatinous  ball,  on  which  both  the  adjacent  vertebrae  play  in 
the  tadpole,  becomes  gradually  more  solid,  and  is  converted 
into  cartilage.     This  cartilage  afterwards  becomes  united  by 
its  anterior  surface  to  the  vertebra  which  is  in  front  of  it; 
and  the  whole  then  becomes  ossified,  so  as  to  compose  only 
one  bone,  its  posterior  surface  remaining  distinct,  and  con- 
tinuing to  play  within  the  cup-like  hollow  of  the  vertebra 
which  is  behind  it.     The  cartilaginous  coccygeal  vertebrae 
of  the  tadpole  are  lost  long  before  there  is  time  for  their 
being  ossified;  but  those  nearest  to  the  body  are  consolidated 
into  one  long  and  straight  os  coccygis,  whicii,  being  joined 
to  the  sacrum  at  an  angle,  gives  rise  to  the  strange  deformi- 
ty observable  at  that  part  of  the  back  of  a  frog;  for  it  here 
looks  as  if  it  had  been  broken.     The  spinal  cavity  is,  at  the 
same  time,  obliterated;  that  j)ortion  of  the  spinal  marrow 
which  had  passed  through  it,  in  the  aquatic  life  of  the  ani- 
mal, being  now  withdrawn.  * 

The  theory  of  the  spinal  origin  of  the  cranial  bones  re- 
ceives considerable  support  from  their  structure  and  relative 
position  in  the  skeleton  of  the  frog.  The  cavity  for  the 
lodgement  of  the  brain,  which  is  enclosed  by  these  vertebrae, 
is  perfectly  continuous,  in  the  same  line  with  the  spinal  ca- 


308  THE  MECHANICAL  FUNCTIONS. 

nal,  which,  indeed,  it  scarcely  exceeds  in  its  diameter.  The 
bones  of  the  face,  are,  at  the  same  time,  expanded  laterally, 
so  as  to  bear  no  proportion  to  the  cranial  cavity.  The  head 
plays  on  the  vertebral  column  by  two  lateral  articular  sur- 
faces, formed  upon  the  root  of  each  leaf  of  the  occipital  bone, 
while  its  body,  or  basilar  portion,  is  scarcely  connected  with 
the  first  cervical  vertebra,  and  has  no  articular  surface. 

In  place  of  ribs,  we  find  only  small,  slender,  detached 
bones,  or  rather  cartilages,  affixed  to  the  extremities  of  the 
transverse  processes  of  some  of  the  vertebras.  They  may 
be  regarded  as  rudi mental  ribs.* 

The  pelvis  consists  of  two  slender  and  elongated  iliac  bones, 
which  are  extended  backwards,  and  which,  at  their  anterior 
extremities,  merely  touch  the  points  of  the  transverse  pro- 
cesses of  the  last  vertebra  of  the  back.  This  vertebra  is  much 
broader  than  the  rest,  and,  although  it  consists  but  of  a  single 
vertebra,  must  be  considered  as  a  sacrum.  The  two  pubic  and 
ischiatic  bones  are  exceedingly  small,  but  still  contribute  to 
form  the  acetabulum,  or  cavity  for  the  reception  of  the  thigh 
bone,  at  the  hinder  extremity  of  the  slender  bones  above 
mentioned.  This  is  the  simplest  possible  form  to  which  the 
pelvis  can  be  reduced,  while  it  preserves  its  attachments  to 
the  spine.  It  presents,  in  this  respect,  a  more  advanced 
stase  of  development  than  that  of  fishes. 

The  connexion  of  the  bones  of  the  anterior  extremities 
with  the  spine  is  analogous  to  that  which  takes  place  in  rays 
and  sharks:  there  being  an  osseous  belt  formed  by  the  sca- 
pula, clavicle,  and  coracoid  bone,  with  the  latter  of  which 
the  humerus  is  connected.  The  sternum  is  large  and  con- 
siderably developed;  making  some  slight  approach  to  the 
expansion  it  receives  in  the  Chelonia.  The  radius  and  ulna 
are  united  into  oneiDone:  the  bones  of  the  arm  and  leg,  in 

*  The  plan  of  reproduction  in  these  animals  requires  that  the  ovary,  or  or- 
ffan  which  contains  the  eggs,  should  be  capable  of  enormous  dilatation,  in 
order  to  contain  the  immense  bulk  to  which  these  eggs  are  expanded,  pre- 
viously to  their  being  brought  fortli.  It  was  probably  in  order  to  make  room 
for  this  dilated  ovary  that  the  ribs  have  not  been  developed. 


PROGRESSIVE  MOTION  IN  BATRACIIIA.  309 

general,  resemble,  in  their  figure  and  connexions,  those  of 
the  higher  orders  oi  Mammalia,  to  the  type  of  which  this 
order  of  reptiles  is  evidently  making  an  approximation. 
There  are  five  toes  in  the  foot,  with  sometimes  the  rudiment 
of  a  sixth:  the  anterior  extremity  has  only  four  toes,  which 
are  without  claws. 

The  necessity  of  employing  the  same  instruments  for  pro- 
gression in  the  water  and  on  land,  is  probably  the  cause 
which  prevents  their  having  the  form  best  adapted  for  ei- 
ther function.  The  hind  feet  of  the  frog,  being  well  con- 
structed for  striking  the  water  backwards  in  swimming,  are, 
in  consequence,  less  capable  of  exerting  a  force  sufficient  to 
raise  and  support  the  weight  of  the  body  in  walking:  and 
this  animal  accordingly  is  exceedingly  awkward  in  its  at- 
tempt to  walk.  On  a  short  level  plane  it  can  proceed  only 
by  leaps;  an  action  which  the  length  and  great  muscularity 
of  the  hind  legs  particularly  fit  them  for  performing.  The 
toad,  on  the  other  hand,  whose  hind  legs  are^hort  and  fee- 
ble, walks  better,  but  does  not  jump  or  swim  so  well  as  the 
frog."^  The  Hyla,  or  tree-frog,  has  the  extremities  of  each 
of  its  toes  expanded  into  a  fleshy  tubercle,  approaching  in 
the  form  of  its  concave  surface  to  that  of  a  sucker,  and  by 
the  aid  of  which  it  fastens  itself  readily  to  the  branches  of 
trees,  which  it  chiefly  inhabits,  and  along  which  it  runs  with 
great  agility.  * 

The  Salamander  is  an  animal  of  the  same  class  as  the 
frog,  undergoing  the  same  metamorphoses  from  the  tadpole 
state.  It  differs  much,  however,  in  respect  to  the  develop- 
ment of  particular  parts  of  the  skeleton.     The  anterior  ex- 

•  It  is  singular  that  the  frog",  though  so  low  in  the  scale  of  vertebratcd  ani- 
mals, should  bear  a  striking  resemblance  to  the  human  conformation  in  its  or- 
gans of  progressive  motion.  This  ai-ises  from  the  exertions  which  it  makes 
in  swimming  being  similar  to  those  of  man  in  walking,  in  as  far  as  they  both 
result  from  the  strong  action  of  the  extensors  of  the  feet.  Hence,  we  find  a 
distinct  calf  in  the  legs  of  both,  produced  by  the  swelling  of  similar  muscles. 
The  muscles  of  the  thigh  present,  also,  many  analogies  with  those  of  man; 
particularly  in  the  presence  of  the  long  muscle  called  the  sariorius,  the  use 
of  which  is  to  turn  the  foot  outwards,  both  in  stepping  and  in  swimming. 


310  THE  MECHANICAL   FUNCTIONS. 

tremitles  of  the  salamander  make  their  appearance  earlier 
than  the  hind  legs,  and  the  tail  remains  as  a  permanent  part 
of  the  structure.  The  rudimental  ribs  are  exceedingly  small, 
and  the  sternum  continues  cartilaginous.  The  pelvis  has  no 
osseous  connexion  with  the  spine,  but  is  merely  suspended 
to  it  by  ligaments.  The  land  salamanders  have  a  rounded 
tail,  but  the  aquatic  species,  or  Tritons,  have  it  compressed 
vertically;  thus  retaining  the  fish-like  form  of  the  tadpole, 
and  the  same  radiated  disposition  of  the  muscles. 

§  3.   Ophidia, 

In  the  class  of  serpents  we  see  exemplified  the  greatest 
possible  state  of  simplicity  to  which  a  vertebrated  skeleton 
can  be  reduced;  for,  as  may  be  seen  in  Fig.  201,  which 
shows  the  skeleton  of  a  viper,  it  consists  merely  of  a  length- 
ened spinal  column,  with  a  head  but  little  developed,  and  a 
series  of  ribs;  but  apparently  destitute  of  limbs,  and  of  the 


bones  which  usually  connect  those  limbs  with  the  trunk; 
there  being  neither  sternum,  nor  scapula,  nor  pelvis.^     In 

*  Professor  Mayer  has,  however,  traced  obscure  rudiments  of  pelvic  bones 
in  the  Unguis  fragilis,  the  Anguis  ventralis,  and  the  Typhlops  avcotatus, 
and  is  of  ophiion  that  they  exist  much  more  generally  in  this  order  of  rep- 
tiles than  lias  been  commonly  imag-ined.  Some  serpents,  as  the  Boa,  Python, 
Tortryx  and  Eryx,  have  claws,  which  may  be  considered  as  rudiments  of  feet, 
visible  externally.     Ln  others,  as  the  Jnguis,  Typhlops,  and  Amphisbocna, 


SERPENTS. 


311 


the  conformation  of  the  skull  and  bones  of  the  fiice,  tlicy  pre- 
sent strong  analogies  with  batrachian  reptiles,  and  also  with 
fishes,  one  tribe  of  which,  namely,  theapodous  oranguilliform 
fishes,  they  greatly  resemble  by  the  length  and  flexibility  of 
the  spine.  These  peculiarities  of  conformation  may  be  in  a 
great  measure  traced  to  the  mode  of  life  for  w^hich  they  are 
destined.  The  food  assigned  to  them  is  living  prey,  which 
they  must  attack  and  vanquish  before  they  can  convert  it  into 
nourishment.  The  usual  mode  in  which  the  boa  seizes  and  de- 
stroys its  victims  is  bycoilingthe  hinder  part  ofits  body  round 
the  trunk  or  branch  of  a  tree,  keeping  the  head  and  anterior 
half  of  the  body  disengaged;  and  then,  by  a  sudden  spring, 
fasten  upon  the  defenceless  object  of  its  attack,  and  twining 
round  its  body  so  as  to  compress  its  chest,  and  put  a  stop  to 

they  exist  concealed  under  the  skhi.  In  others,  he  has  discovered  cartilagi- 
nous filaments,  which  he  conceives  to  correspond  to  these  parts.  (Annales 
des  Sciences  Naturelles,  VII.  170.)  Some  of  these  are  represented  in  the 
following  figures.     Fig.  203  exhibits  the  claw  of  the  Boa  constrictor y  placed 


203 


205        206 


2or 


209 


at  the  termination  of  a  series  of  bones,  representing  very  imperfectly  the 
bones  of  the  lower  extremities.  Fig.  204  shows  the  muscles  attached  to 
these  small  bones.  The  three  following  figures,  205,  206,  and  207,  repre- 
sent the  claws  and  rudi mental  bones  of  the  Tortrix  scijtak,  Tortrix  coral- 
linus,  ^Wil  Anguis  fragilis,  respectively.  Tliose  of  the  Jmphisbaena  alba,  Fig. 
208,  and  the  Coluber  pullatus.  Fig.  209,  are  still  less  developed.  The  Clial- 
cides,  or  snake  lizard,  which  has  four  minute  feet,  is  represented  in  Fig.  210. 


312 


THE  MECHANICAL  FUNCTIONS. 


its  respiration.  Venomous  serpents,  on  the  other  hand,  coil 
themselves  into  the  smallest  possible  space,  and  suddenly 
darting  upon  the  unsuspecting  or  fascinated  straggler,  inflict 
the  quickly  fatal  wound.* 

It  is  evident,  from  these  considerations,  that,  in  the  ab- 
sence of  all  external  instruments  of  prehension  and  of  pro- 
gressive motion,  it  is  necessary  that  the  spine  should  be 
rendered  extremely  flexible,  so  as  to  adapt  itself  to  a  great 
variety  of  movements.  This  extraordinary  flexibility  is 
given,  first,  by  the  subdivision  of  the  spinal  column  into  a 
great  number  of  small  pieces;  secondly,  by  the  great  free- 
dom of  their  articulations;  and  thirdly,  by  the  peculiar  mo- 
bility and  connexions  of  the  ribs. 

Numerous  as  are  the  vertebrae  of  the  eel,  the  spine  of 
which  consists  of  above  a  hundred,  that  of  serpents  is  in 
general  formed  of  a  still  greater  number.  In  the  rattle- 
snake [Crotahcs  horridus)  there  are  about  two  hundred; 

and  above  three  hundred  have 
been  counted  in  the  spine  of 
the  Coluber  natrix.  These 
vertebrae  are  all  united  by  ball 
and  socket  joints,  as  in  the 
adult  batrachia;  the  posterior 
rounded  eminence  of  each  ver- 
tebra being  received  into  the 
anterior  surface  of  the  next. 
Fig.  202  is  a  view  of  this  por- 
tion of  the  skeleton  in  the  Boa 
constrictor,  showing  the  arti- 
culation of  the  ribs  with  the 
vertebrae. 

While  provision  has  thus  been  made  for  extent  of  mo- 
tion, extraordinary  care  has  at  the  same  time  been  bestou'ed 
upon  the  security  of  the  joints.    Thus,  we  find  them  efiectu- 

*  Their  prey  is  swallowed  entire;  and  therefore,  as  we  shall  afterwards  find, 
the  bones  of  the  jaws  and  face  are  formed  to  admit  of  great  expansion,  and 
of  great  freedom  of  motion  upon  one  another. 


SERPENTS.  313 

ally  protected  from  dislocation  by  the  locking;  in,  above 
and  below,  of  the  articular  processes,  and  by  the  close  in- 
vestment of  the  capsular  ligaments.  The  direction  of  the 
surfaces  of  these  processes,  and  the  shape  and  length  of  the 
spinous  processes,  are  such  as  to  allow  of  free  lateral  flex- 
ion, but  to  limit  the  vertical  and  longitudinal  motions:  and 
whatever  degree  of  freedom  of  motion  may  exist  between 
the  adjoining  vertebra?,  that  motion  being  multiplied  along 
the  column,  the  flexibility  of  the  whole  becomes  very  great, 
and  admits  of  its  assuming  every  degree  and  variety  of 
curvature.  The  presence  of  a  sternum,  restraining  the  mo- 
tions of  the  ribs,  would  have  impeded  all  these  movements, 
and  would  have  also  been  an  insurmountable  bar  to  the  di- 
latation of  the  stomach,  which  is  rendered  necessary  by  the 
habit  of  the  serpent  of  gorging  its  prey  entire. 

The  mode  in  which  the  boa  exerts  a  powerful  pressure  on 
the  bodies  of  the  animals  it  has  seized,  and  which  it  has  en- 
circled within  its  folds,  required  the  ribs  to  be  moveable  la- 
terally, as  well  as  backwards,  in  order  to  elude  the  force 
thus  exerted.  The  broad  convex  surfaces  on  which  they 
play  give  them,  in  this  respect,  an  advantage  which  the  or- 
dinary mode  of  articulation  would  not  have  aflforded.  The 
spinous  processes  in  this  tribe  of  serpents  are  short  and  wide- 
ly separated,  so  as  to  allow  of  flexion  in  every  direction.  In 
the  rattle-snake,  on  the  other  hand,  their  length  and  oblique 
position  are  such  as  to  limit  the  upward  bending  of  the  spinal 
column,  although,  in  other  respects,  its  motion  is  not  restricted. 
The  vertebrae  at  the  end  of  the  tail  are  furnished  with  broad 
transverse  processes  for  the  attachment  of  the  first  joints  of 
the  rattle. 

But  of  whatever  variety  of  flexions  we  may  suppose  the 
lengthened  body  of  a  serpent  to  be  capable,  it  will,  at  first 
view,  be  diflacult  to  conceive  how  these  simple  actions  can 
be  rendered  subservient  to  the  purposes  of  progression  on 
land :  and  yet  experience  teaches  us  that  few  animals  advance 
with  more  celerity  on  the  surface  of  the  ground,  or  dart  upon 
their  prey  with  greater  promptitude  and  precision.     Tlifiy 

Vol.  I.  40 


314  THE  MECHANICAL  FUNCTIONS. 

raise  themselves  without  difficulty  to  the  tops  of  the  highest 
trees,  and  escape  to  their  hiding  places  with  a  quickness 
which  eludes  observation  and  baffles  the  efforts  of  their  pur- 
suers. 

The  solution  of  this  enigma  is  to  be  sought  for  partly  in 
the  structure  of  the  skin,  which,  in  almost  every  species,  is 
covered  with  numerous  scales:  and  partly  in  the  peculiar 
conformation  of  the  ribs.  The  edges  of  the  scales  form  rough 
projections,  which  are  directed  backwards,  so  as  to  catch  the 
surfaces  of  the  bodies  to  which  they  are  appUed,  and  to  pre- 
vent any  retrograde  motion.  In  some  species,  the  integu- 
ment is  formed  into  annular  plates,  reminding  us  of  the  struc- 
tures so  prevalent  among  worms  and  myriapode  animals. 
Each  scale  is  connected  with  a  particular  set  of  muscular 
fibres,  capable  of  raising  or  depressing  it,  so  that,  in  this  way, 
it  is  converted  into  a  kind  of  toe ;  and  thus  the  body  rests 
upon  the  ground  by  numerous  fixed  points  of  support. 

This  support  is  farther  strengthened  by  the  connexion  of 
the  ribs  with  the  abdominal  scuta,  or  the  scales  on  the  under 
side  of  the  body.     The  mode  in  which  the  ribs  become  aux- 
iliary instruments  of  progressive  motion  was  first  noticed  by  Sir 
Joseph  Banks.*     Whilst  he  was  watching  the  movements  of  a 
Coluber  of  unusual  size  which  was  exhibited  in  London,  and 
was  moving  briskly  along  the  carpet,  he  thought  he  saw  the  ribs 
come  forward  in  succession,  like  the  feet  of  a  caterpillar.     Sir 
Everard  Home,  to  wham  Sir  Joseph  Banks  pointed  out  this 
circumstance,  verified  the  fact  by  applying  his  hand  below 
the  serpent,  and  he  then  distinctly  felt  the  ends  of  the  ribs 
moving  upon  the  palm,  as  the   animal  passed  over  it.     The 
mode  in  which  the  ribs  are  articulated  with  the  spine  is  pe- 
culiar, and  has   evidently  been   employed  with  reference  to 
this  particular  function  of  the  ribs,  which  here  stand  in  place 
of  the  anterior  and  posterior  extremities,  possessed  by  most 
vertebrated  animals,  and  characterizing  the  type  of  their  os- 
seous fabric.     In  the  ordinary  structure,  the  head  of  each 
rib  has  a  convex  surface,  that  plays  either  on  the  body  of  a 

♦  Philos,  Trans,  for  1812,  p.  163. 


PROGRESSIVE  MOTIOlff  IN  SERPENTS.  315 

single  vertebra  with  which  it  is  connected,  or  upon  the  two 
bodies  of  adjacent  vertebrae :  but  in  serpents  the  extremity 
of  the  head  of  the  rib  has  two  slightly  concave  articular  sur- 
faces, which  play  on  a  convex  protuberance  of  the  vertebra. 
This  structure  is  attended  with  the  advantage  of  preventing 
the  ribs  from  interfering  with  the  motions  of  the  vertcbrse 
upon  one  another.     At  their  lower  ends  the  ribs  of  one  side 
have  no  connexion  with  those  of  the  other,  nor  are  they 
joined  to  any  bone  analogous  to  a  sternum :  for,  except  in  the 
Ophiosaiirus  and  the  Blind-worm  {Anguis  fragilis,)  there  is  no 
vestige  either  of  a  sternum  or  scapula,  in  any  animal  of  this 
class.     Each  rib  terminates  in  a  slender  cartilage,  tapering 
to  a  point,  which  rests,  for  its  whole  length,  upon  the  upper 
surface  of  one  of  the  scuta,  or  broad  scales  on  the  lower  side 
of  the  body.     These  scuta,  which  are  thus  connected  with 
the  ends  of  the  ribs,  and  which  are  moved  by  means  of  short 
muscles,  may  be  compared  to  hoofs,  while  the  ribs  themselves 
may  be  considered  as  performing  the  office  of  legs.     The 
ribs  move  in  pairs ;  and  the  scutum  under  each  pair,  being 
carried  along  with  it  in  all  its  motions,  and  laying  hold  of  the 
ground  by  its  projecting  edge,  becomes  a  fixed  point  for  the 
advance  of  the  body.     This  motion.  Sir  E.  Home  observes, 
is  beautifully  seen  when  a  snake  is  climbing  over  an  angle  to 
get  upon  a  flat  surface.     When  the  animal  is  moving  on  a 
plane,  it  alters  its  shape  from  a  circular  or  oval  form,  to  one 
that  approaches  to  a  triangle,  of  which  the  surface  applied 
to  the  ground  forms  the  base.     Five  sets  of  muscles  are  pro- 
vided for  the  purpose  of  giving  to  the  ribs  the  motions  back- 
wards and  forwards,  by  which,  as  levers,  they   effect  this 
species  of  progression.     These  muscles  are  disposed  in  regular 
layers ;  some  passing  over  one  or  two  ribs  to  be  attached  to 
the  succeeding  rib.     In  all  snakes  the  ribs  are  continued 
backwards  much  beyond  the  region  occupied  by  the  lungs; 
and  although  the  anterior  set  are  subservient  to  respiration, 
as  well  as  to  progressive  motion,  it  is  evident,  that  all  those 
posterior  to  the  lungs  must  be   employed  solely  for  the  latter 
of  these  purposes. 


316  THE  MECHANICAL  FUNCTIONS. 

It  is  easy  to  understand  how  the  serpent  can  slowly  ad- 
vance, by  this  creeping,  or  vermicular  motion,  consisting  in 
reality  of  a  succession  of  very  short  steps.     But  its  progress 
is  accelerated  by  the  curvatures  into  which  it  throws  its 
body;  the  fore  part  being  fixed,  and  the  hind  part  brought 
near  to  it;  then,  by  a  reverse  process,  the  hind  part  is  fixed, 
and  the  head  projected  forwards.     By  an  alteration  of  these 
movements,  assisted  by  the  actions  of  the  ribs,  the  serpent 
is  enabled  to  glide  onwards  with  considerable  rapidity,  and 
without  attracting  observation.     But  where  greater  expedi- 
tion is  necessary,  they  employ  a  more  hurried  kind  of  pace, 
although  one  which  exposes  them  more  to  immediate  view. 
The  body,  instead  of  being  bent  from  side  to  side,  is  raised 
in  one  great  arch,  of  which  the  two  extremities  alone  touch 
the  ground;  and  these  being  alternately  employed  as  points 
of  support,  are  made  successively  to  approach  and  to  sepa- 
rate from  each  other,  the  body  being  propelled  by  bringing 
it  from  a  curved  to  a  straight  line. 

There  is  yet  a  third  kind  of  motion,  which  serpents  oc- 
casionally resort  to,  when  springing  upon  their  prey,  or 
when  desirous  of  making  a  sudden  escape  from  danger. 
They  coil  themselves  into  a  spiral,  by  contracting  all  the 
muscles  on  one  side  of  the  body,  and  then,  suddenly  throw- 
ing into  violent  action  all  the  muscles  on  the  opposite  side, 
the  whole  body  is  propelled,  as  if  by  the  release  and  un- 
winding of  a  powerful  spring,  with  an  impulse  which  raises 
it  to  some  height  from  the  ground,  and  projects  it  to  a  con- 
siderable distance. 

Thus  these  animals,  to  which  nature  has  denied  all  exter- 
nal members,  are  yet  capable,  by  the  substitution  of  a  differ- 
ent kind  of  mechanism,  still  constructed  from  the  elements 
belonging  to  the  primitive  type  of  vertebrated  animals,  of 
silently  gliding  along  the  surface  of  the  earth,  of  creeping 
^up  trees,  of  striding  rapidly  across  the  plain,  and  of  exe- 
cuting leaps  w^ith  a  vigour  and  agility  which  astonish  the 
beholder,  and  which,  in  ages  of  ignorance  and  superstition, 
were  easily  ascribed  to  supernatural  agency. 


SAURIAN  REPTILES.  317 


§  4.  Sauria. 

The  conformation  of  those  parts  of  the  frame  which  are 
subservient  to  progressive  motion  becomes  more  perfect  in 
the  class  of  Saurian  reptiles,  which  includes  all  the  Lizard 
tribes.     Several  links  of  connexion  with  the  preceding  class 
may  still  be  noticed,  marking  the  progress  of  development, 
as  we  follow  the  ascending  series  of  animals.     Rudiments 
of  the  bones  of  the  extremities,  and,  also,  of  the  sternum, 
make  their  appearance  very  visibly  in  the   Ophiosauriis, 
and  in  the  blind  worm,  {*Bnguis  fragilis.)     The  Siren  la- 
certina  has  two  diminutive  fore  feet,  placed  close  to  the 
head.    The  Lacerta  lumbricoides  of  Linnaeus,  or  the  Bipes 
canaliculatus  of  Lacepede,  which  is  found  in  Mexico,  and 
of  which  a  specimen  is  preserved  in  the  collection  at  Paris, 
has  a  pair  of  very  short  feet,  also  placed  near  the  head,  and 
divided  into  four  toes,  with  the  rudiment  of  a  fifth.     The 
Lacerta  bipes  (Linn.)  or  Sheliopiisic  of  Pallas,  has,  on  the 
other  hand,  a  pair  of  hind  feet  only,  but  extremely  small,  to- 
gether with  rudiments  of  a  scapula  and  clavicle  concealed 
under  the  skin.     Next  in  order  must  be  placed  the  Chal- 
cides,  or  Snake-lizard,  (Fig.  210,)  and  the  Lacerta  seps,  ani- 
mals frequently  met  with  in  the  South  of  France,  and  which 
have  four  minute  feet,  totally  inefficient  for  the  support  of  the 
body,  and  only  remotely  useful  in  contributing  to  its  pro- 
gressive undulations. 

Ascending  from  these,  we  may  form  a  series  of  reptiles, 
in  which  the  development  of  the  limbs  becomes  more  and 
more  extended,  till  we  arrive  at  Crocodiles,  in  which  they 
attain  a  considerable  degree  of  perfection.  As  a  consequence 
of  this  greater  development  of  the  skeleton,  we  find  the 
trunk  divisible  into  separate  regions.  We  now,  for  the  first 
time,  meet  with  a  distinct  neck,  separating  the  head  from  the 
thorax,  which  is  itself  distinguishable  from  the  abdomen; 
and  a  distinct  sacrum  is  interposed  between  the  lumbar  and 
the  caudal  vertebrae. 


318  THE  MECHANICAL  TUNCTIONS. 

A  farther  approach  to  the  higher  classes,  is  observable  in 
the  number  of  cervical  vertebrae,  which  is  almost  constantly 
seven;  as  we  shall  find  it  to  be  in  the  mammalia.  The  arti- 
culations of  the  vertebrae  are  similar  to  those  of  serpents,  in- 
asmuch as  they  consist  of  ball  and  socket  joints.  In  that  of 
the  occipital  bone  with  the  first  vertebra  of  the  neck,  we  find 
that  nature  again  reverts  to  the  simpler  form  of  a  single  con- 
dyle projecting  from  the  body  of  the  occipital  bone,  instead 
of  lateral  condyles  proceeding  from  its  leaves,  as  we  noticed 
was  the  structure  in  the  batrachia.  The  caudal  vertebrae  are 
always  numerous,  and  the  tail  is  compressed  vertically, 
which  is  the  form  most  favourable  for  progression  in  water. 
They  are  remarkable,  also,  for  having  inferior  spinous  pro- 
cesses attached  to  the  bodies  by  cartilages;  a  structure  ana- 
logous to  that  which  we  have  seen  in  fishes. 

The  number  of  ribs  differs  in  different  species  of  Sauria: 
they  are  always  articulated  to  the  extremities  of  the  trans- 
verse processes  of  the  vertebrae,  of  which  they  appear  to  be 
continuations.  Processes  of  this  description  also  occur  in 
the  neck,  attached  to  the  transverse  processes  of  the  cervical 
vertebrae;  and  these  have  been  regarded  as  cervical  ribs. 
Their  presence  are  impediments  to  the  flexions  of  the  neck; 
whence  arises  the  difficulty  which  the  crocodile  appears  to 
have  in  bending  the  neck,  while  turning  round  upon  the  ani- 
mal he  is  pursuing.  In  the  thorax,  the  ribs  are  connected 
with  a  broad  sternum;  but  there  are  other  ribs,  both  before 
and  behind,  which  have  no  such  termination,  and  therefore 
bear  the  name  oi  false  ribs. 

The  pelvis  consists  chiefly  of  the  iliac  bones,  which,  as 
in  the  batrachia,  pass  backwards  to  form  the  articular  cavity 
for  the  thigh  bone.  Two  small  and  slender  bones  extend 
forwards  from  the  pubic  bones,  on  the  under  side  of  the 
body,  apparently  for  the  purpose  of  supporting  the  abdomi- 
nal viscera.*  The  bones  of  the  extremities  are  very  perfectly 
formed,  approaching  in  their  shape  and  arrangement  very 

*  They  appear  to  be  analogous  to  the  marsupial  bones  peculiar  to  a  family 
of  mammalia. 


FEET  or  THE  GECKO. 


31S 


nearly  to  the  corresponding  parts  of  the  skeleton  of  the 
higher  orders  of  quadrupeds.  The  toes  are  usually  provided 
with  membranes  spread  between  them,  to  assist  in  swim- 
ming. The  form  of  the  tail,  which  is  generally  compressed 
vertically,  like  that  of  fishes,  though  perhaps  not  to  an  equal 
degree,  is  another  indication  of  their  being  formed  for  an 
aquatic  life:  for  where  the  tail  has  this  shape,  we  always  find 
that  the  chief  muscular  power  is  bestowed  upon  it  as  an  in- 
strument of  aquatic  progression,  producing,  by  its  lateral 
flexions,  a  horizontal  movement  of  the  body.  Crocodiles 
and  alligators,  for  instance,  which  have  this  conformation, 
are  comparatively  weak  when  on  land,  and  as  soon  as  they 
have  seized  their  prey,  their  efforts  are  always  directed  to 
drag  it  with  them  into  the  water;  knowing  that  when  in 
their  own  element  they  can  readily  master  its  struggles,  and 
dispose  of  it  as  they  please. 

In  the  Gecko  tribe,  we  find  a  particular  mechanism  pro- 
vided for  effecting  the  adhesion  of  the  feet  to  the  objects  to 
which  they  are  applied.  It  is  somewhat  analogous  to  that 
employed  in  the  case  of  the  house-fly,  already  mentioned. 
Each  foot  has  five  toes;  all,  except  the  thumb,  terminated 
by  a  sharp  curved  claw.  On  the  under  surface  of  each  toe 
(represented  in  Fig.  211)  there  are  as  many  as  sixteen  trans- 
212  211       verse  slits,  leading  to  the  same 

number  of  cavities,  or  sacsj 
these  open  forwards,  and  their 
external  edge  is  serrated,  ap- 
pearing like  the  teeth  of  a  small- 
toothed  comb.  A  section  of 
the  foot,  showing  these  cavi- 
ties, is  seen  in  Fig.  212.  All 
these  parts,  together  with  the 
cavities  are  covered  or  lined 
with  cuticle.  Below  them  are 
large  muscles  wh  ich  draw  do  wn 
the  claw;  and  from  the  tendons 
of  these  muscles  arise  two  sets  of  smaller  muscles,  situated  so 


320  THE    MECHANICAL  FUNCTIONS. 

as  to  be  put  upon  the  stretch,  when  the  former  are  in  action. 
By  the  contractions  of  these  muscles,  the  orifices  of  the  cavi- 
ties, or  sacs  to  which  they  belong,  are  opened,  and  the  serrated 
edges  applied  accurately  to  the  surfaces  with  which  the  feet 
are  in  contact.  Sir  Everard  Home,  in  his  account  of  this 
structure,  compares  it  to  the  sucking  disk  of  the  Remora.^ 
By  its  means  the  animal  is  enabled  to  walk  securely  upon 
the  smoothest  surfaces,  even  in  opposition  to  the  tendency 
of  gravity.  It  can  run  very  quickly  along  the  walls  or  ceil- 
ing of  a  building,  in  situations  where  it  cannot  be  supported 
by  the  feet,  but  must  depend  altogether  upon  the  suspension 
derived  from  a  succession  of  rapid  and  momentary  adhe- 
sions. 

Although  the  Sauria  are  better  formed  for  progressive 
motion  than  any  of  the  other  orders  of  reptiles,  yet  the 
greater  shortness  and  oblique  position  of  their  limbs,  com- 
pared with  those  of  mamrniferous  quadrupeds,  obliges  them 
in  o-eneral  to  rest  the  weight  of  the  trunk  of  the  body  on 
the  ground,  when  they  are  not  actually  moving.  None  of 
these  reptiles  have  any  other  kind  of  pace  than  that  of  walk- 
ing, or  jumping;  being  incapable  of  performing  either  a  trot 
or  a  gallop,  in  consequence  of  the  obliquity  of  the  plane  in 
which  their  limbs  move.  The  Chameleon  walks  with  great 
slowness  and  apparent  difficulty;  and  we  have  seen  that,  in 
consequence  of  the  structure  of  the  bones  of  its  neck,  the 
Crocodile^  though  capable  of  swift  motion  in  a  straight  line, 
is  unable  to  turn  itself  round  quickly.  The  general  type 
of  these  reptiles,  having  reference  to  an  amphibious  life,  has 
not  attained  that  exclusive  adaptation  to  a  terrestrial  exist- 
ence, which  we  find  in  the  higher  orders  of  the  Mammalia. 
But  before  proceeding  to  consider  these,  we  have  to  notice 
a  sino-ular  group  of  animals,  whose  conformation  appears  to 
be  exceedingly  anomalous,  and  as  if  it  interrupted  the  regu- 
larity of  the  ascending  series,  of  which  it  seems  to  be  a  col- 
lateral ramification. 

*  Philosophical  Transactions  for  1816,  p.  151,  and  323. 


CHELONIAN  REPTILES.  321 


§  5.   Chelonia, 

The  order  of  Chelonian  Peptiles,  which  comprises  all 
the  tribes  of  Tortoises  and  Turtles,  appears  to  constitute  an 
exception  to  the  general  laws  of  conformation,  which  pre- 
vail among  Vertebrated  Animals:  for  instead  of  presenting 
a  skeleton  wholly  internal,  the  trunk  of  the  body  is  found 
to  be  enclosed  on  every  side  in  a  bony  case,  which  leaves 
openings  only  for  the  head,  the  tail,  and  the  fore  and  hind 
extremities.  That  portion  of  this  osseous  expansion  which 
covers  the  back  is  termed  the  Carapace;  and  the  flat  plate 
which  defends  the  lower  part  of  the  body  is  termed  the 
plastron.  It  is  a  form  of  structure  that  reminds  us  of  the 
defence  provided  for  animals  ver}''  low  in  the  scale  of  or- 
ganization, such  as  the  echinus,  the  Crustacea,  and  the  bi- 
valve mollusca.  Yet  the  substance  which  forms  these  strong 
bucklers,  both  above  and  below,  is  a  real  osseous  structure, 
developed  in  the  same  manner  as  other  bones,  subject  to  all 
the  changes,  and  having  all  the  properties  of  these  struc- 
tures. The  great  purpose  which  nature  seems  to  have  had 
in  view  in  the  formation  of  the  Chelonia  is  security;  and  for 
the  attainment  of  this  object  she  has  constructed  a  vaulted 
and  impenetrable  roof,  capable  of  resisting  enormous  pres- 
sures from  without,  and  proof  against  any  ordinary  mea- 
sures of  assault.  It  is  to  the  animal  a  strong  castle,  into 
which  he  can  retire  on  the  least  alarm,  and  defy  the  efforts 
of  his  enemies  to  dislodge  or  annoy  him. 

These  considerations  supply  us  with  a  key  to  many  of 
those  apparent  anomalies,  which  cannot  fail  to  strike  us  in 
viewing  the  dispositions  of  the  parts  of  the  skeleton  (Fig. 
213,)  and  the  remarkable  inversion  they  appear  to  have  un- 
dergone, when  compared  with  the  usual  arrangement.  We 
find,  however,  on  a  more  attentive  examination,  that  all  the 
bones  composing  the  skeleton  in  other  vertebrated  animals 
exist  also  in  the  tortoise;  and  that  the  bony  case  which  en- 
velops all  the  other  parts  is  really  formed  by  an  extension 

Vol.  I.  41 


322 


THE  MECHANICAL  FUNCTIONS. 


of  the  spinous  processes  of  the  vertebrae  and  ribs  on  the  one 
side,  and  of  the  usual  pieces  which  compose  the  sternum  on 
the  other.  The  upper  and  lower  plates  thus  formed  are 
united  at  their  edges  by  expansions  of  the  sternocostal  ap- 
pendices, which  become  ossified.  Thys,  no  new  element 
has  been  created;  but  advantage  has  been  taken  of  those  al- 
ready existing  in  the  general  type  of  the  vertebrata,  to  mo- 


dify their  forms,  by  giving  them  different  degrees  of  rela-' 
tive  development,  and  converting  them,  by  these  trans- 
formations, into  a  mechanism  of  a  very  different  kind,  and 
subservient  to  other  objects  than  those  to  which  they  are 
usually  applied.  It  is  scarcely  possible  to  have  stronger" 
proofs,  if  such  were  wanting,  of  the  unity  of  plan  which  has 
regulated  the  formation  of  all  animal  structures,  than  those 
afforded  by  the  skeleton  of  the  tortoise. 

The  first  step  taken  to  secure  the  relative  immobility  of 
the  trunk,  is  to  unite  in  one  rigid  bony  column  all  its  verte- 


CIIELONIAN  REPTILES.  323 

brae,  and  to  allow  of  motion  only  in  those  of  the  neck,  and 
of  the  tail.     The  former,  accord ini»;ly,  are  all  anchylosed  to- 
gether, leaving,  indeed,  traces  of  their  original  forms  as  se- 
parate vertebrae,  but  exhibiting  no  sutures  at  the  place  of 
junction.     The  canal  for  the  spinal  marrow  is  preserved,  as 
usual,  above  tlic  bodies  of  these  coalesced  vertcbric,  and  is 
formed  by  their  united  leaves;  the  arches  being  completed 
by  the  spinous  processes.     But  these  processes  do  not  ter- 
minate in  a  crest  as  usual;  they  are  farther  expanded  in  a 
lateral  direction,  forming  flat  pieces  along  the  back,  which 
are  united  to  one  another  by  sutures,  and  which  are  also 
joined  to  the  expanded  ribs,  so  as  to  form  the  continuous 
plane  surface  of  the  carapace.     The  transverse  processes  of 
the  vertebrae  are  well  marked,  but,  though  firmly  united  to 
the  ribs,  do  not  give  rise  to  them;  for  the  ribs,  which  are 
flattened  and  expanded,  so  as  to  touch  one  another  along 
their  whole  length,  are  inserted  below,  between  the  bodies 
of  every  tvvo  adjoining  vertebras;  while  above,  they  are 
united  by  suture  with  the  plates  of  the  spinous  processes. 
This  change  in  the  situation  of  the  ribs  is  the  consequence 
of  the  change  in  their  oflice.     When  designed  to  be  very 
moveable,  we  find  them  attached  either  to  the  extremities  of 
the  transverse  processes,  or  to  the  articular  surfaces  of  a  sin- 
gle vertebra;  but  where  solidity  and  security  are  aimed  at, 
they  are  always  inserted  between  the  bodies  of  two  verte- 
brae.    This  we  shall  find  to  be  the  case  also  in  birds,  where 
the  bones  of  the  thorax  are  required  to  be  immoveable.     It 
is  remarkable,  indeed,  that  a  great  number  of  the  peculiari- 
ties which  distinguish  the  conformation  of  the  chelonia  from 
that  of  other  reptiles,  indicate  an  approach  to  the  structure 
of  birds;  as  if  nature  had  intended  this  small  group  of  ani- 
mals to  be  an  intermediate  link  of  gradation  to  that  new  and 
important  type  of  animals  destined  for  a  very  diflerent  mode 
of  existence. 

The  sterno-costal  appendages,  which  connect  the  ribs  to 
the  sternum,  are,  in  most  animals,  cartilaginous:  thougli  oc- 
casionally we  find  them  partially  ossified.     In  the  tortoise, 


1^- 

324  THE  MECHANICAL  FUNCTIONS. 

however,  their  ossification  is  not  only  complete,  but  has 
been  expanded  laterally,  so  as  to  form  a  continuous  surface 
with  the  extremities  of  the  ribs  and  with  the  edges  of  the 
plastron,  and  completely  to  fill  up  the  vacancy  between 
diem;  constituting  a  dense  and  solid  wall,  which  entirely 
closes  the  sides  of  the  general  bony  case.  So  strong  is  the 
tendency  to  ossification  in  all  these  pieces,  that  the  sutures 
at  first  formed  between  them  are  often,  in  process  of  time, 
obliterated;  and  the  bony  fibres  are  continuous  throughout  a 
ffreat  extent  of  surface. 

The  most  remarkable  metamorphosis  in  the  osseous  sys- 
tem of  this  new  type  is  that  which  occurs  in  the  sternum. 
So  expanded  are  all  its  parts,  that  it  is  difficult  to  recognise 
this  bone  under  the  disguised  form  in  which  it  constitutes 
the  plastron,  or  broad  plate,  which,  as  we  have  seen,  covers 
the  whole  of  the  under  side  of  the  body.  Yet,  by  a  careful 
examination  of  its  structure,  both  in  the  young  animal,  and 
also  in  the  adult,  wiien  the  sutures  are  not  obliterated,  we 
may  easily  recognise  the  nine  elements  of  the  sternum; 
namely,  the  one  in  the  middle  and  fore  part,  and  the  four 
pairs  of  lateral  pieces;  each  having  been  formed  from  its  re- 
spective centre  of  ossification.  In  form  and  relative  propor- 
tion, indeed,  they  are  widely  different  from  the  same  parts 
as  they  are  presented  in  the  skeletons  of  other  animals:  yet 
in  number  and  in  relative  situations  they  preserve  that  con- 
stancy and  uniformity  so  characteristic  of  the  beautiful  har- 
mony which  pervades  all  animal  structures. 

It  is  to  be  noticed,  also,  that  as  the  plates,  which  form 
this  investing  case,  are  bony  structures,  they  could  not  with 
any  safety  have  been  exposed  to  the  action  of  the  atmo- 
sphere. Hence  we  find  them  covered  throughout  with  a  thin 
horny  plate,  originally  a  production  of  the  integument.  It 
is  this  substance  which  is  commonly  known  by  the  name  of 
tortoise  shell.* 

•  It  should  be  observed,  that  the  divisions  of  these  plates,  which  appear 
externally,  bear  no  relation  to  the  sutures  which  scpai'atc  the  subjacent  bones, 


*         CHELONIAN  REPTILES.  ^25 

The  immobilil}-  of  the  trunk  is  compensated,  as  far  as  re- 
gards the  safety  of  the  head,  by  the  great  flexibility  of  the 
neck;  which  is  composed  of  seven  vertebrae,  unencumbered 
by  processes,  and  capable  of  taking  a  double  curvature  like 
the  letter  S,  when  the  head  is  to  be  retracted  within  the  ca- 
rapace. These  vertebrae  are  joined  by  the  ball  and  socket 
articulation  common  to  all  the  existing  species  of  reptiles.* 
The  articulation  of  the  head  with  the  neck  is  effected  in  the 
same  manner;  but  it  is  interesting  to  remark  that  the  occipi- 
tal condyle,  which  is  situated  at  the  lower  margin  of  the 
great  aperture,  though  presenting  a  single  convex  surface, 
225       p^.^__,^__p  yet  has  that  surface  evidently  di- 

vided into  three  parts;  the  two  up- 
per portions  being  lateral,  and  the 
lower  portion  in  the  middle.  These 
three  articular  surfaces  are  seen  im- 
mediately below  the  central  aper- 
ture, F,  in  Fig.  215,  which  exhi- 
bits the  skull  of  the  Testudo  my  das,  viewed  from  behind. 
Although  closely  approximated,  a  faint  line  of  demarcation, 
which  divides  their  surface,  indicates  an  incipient  tendency 
to  separate;  we  shall  find  that,  in  the  farther  steps  of  deve- 
lopment which  occur  in  the  higher  classes,  this  separation 
actually  takes  place  by  the  obliteration  of  the  lower  articu- 
lar surface,  and  the  transfer  of  the  two  lateral  surfaces  to  the 
condyloid  processes  arising   from  the  development  of  the 
leaves  of  the  occipital  bone. 

The  singular  conformation  of  the  bones  of  the  head,  in  the 
turtle,  affords  fresh  evidence  in  support  of  the  theory  that 
these  bones  were  originally  vertebrae.  The  brain  of  the  tor- 
toise is  exceedingly  small;  and,  yet,  the  skull,  when  viewed 
from  above,  presents  an  appearance  of  great  brcadth^as  if  it 
enclosed  a  cavity  of  large  dimensions.     But  if  we  look  upon 

so  that  it  is  not  possible  to  draw  inferences  respecting  the  form  of  the  latter 
from  the  mere  inspection  of  the  external  shell. 

*  The  expression  of  this  fi\ct  is  thus  qualified,  because  it  does  not  apply 
to  many  fossil  or  extinct  species,  such  as  the  Iddhyosaurus. 


326  THE  MECHANICAL  FUNCTIONS. 

it  from  behind,  as  is  shown  in  Fig.  215,  we  soon  discover 
that  the  real  cavity  in  which  the  brain  is  lodged,  and  to 
which  the  aperture  at  f  leads,  is  very  small,  only  just  admit- 
ting the  end  of  the  finger,  and  that  the  broad  plates  of  bone, 
p,  p,  which  form  the  upper  surface  of  the  skull,  have  no  re- 
lation to  this  cavity,  and  are  merely  extended  over  the  tem- 
poral muscles,  which  are  of  very  large  size,  occupying  the 
whole  of  the  spaces  s,  s;  which  spaces  are  completely  sur- 
rounded by  these  bones.  It  would  appear  that  the  same  ten- 
dency to  lateral  expansion,  which  exists  in  the  spinous  pro- 
cesses of  the  dorsal  vertebrae,  prevails,  also,  among  those 
which  contribute  to  form  the  skull.  The  parietal  bones, 
which  represent  the  spinous  processes  of  the  second  cranial 
vertebra,  after  having  performed  their  primary  office  of  pro- 
tecting the  hemispheres  of  the  brain  by  closing  over  them, 
still  proceed  in  their  development,  forming  first  a  crest  on 
the  upper  part  of  the  real  cranium,  and  then  separating  to 
the  right  and  left,  and  expanding  horizontally  into  the  upper 
roof  (p,  p,)  already  mentioned,  for  the  protection  of  the  tem- 
poral muscles.  This  great  breadth  of  the  head  in  the  turtle 
gives  the  animal  an  aspect  of  superior  intelligence,  to  which 
character,  from  the  really  diminutive  size  of  its  brain,  it  is, 
in  no  respect,  entitled.  As  the  turtle  is  unable  to  withdraw 
its  head  \vithin  the  carapace,  such  extraordinary  protection 
appears  to  have  been  necessary:  for  it  is  not  met  with  in  the 
tortoise,  which  has  a  carapace  sufficiently  capacious  to  give 
shelter  to  the  head  whenever  occasion  may  require.* 

This  arrangement  of  the  expanded  spinous  processes  and 
ribs  gives  rise  to  a  singular  inversion  in  the  position  of  the 
scapula;  for  it  is  here  placed  on  the  inside  of  the  ribs  and 
sternum,  that  is,  between  the  carapace  and  plastron.t     The 

*  The  analogy  of  the  spine  of  the  occipital  bone  with  that  of  a  vertebra  is 
farther  shown  by  this  bone  extending-  backwards  to  a  considerable  length, 
exactly  in  the  manner  of  the  spinous  processes  of  the  cervical  vertebrae  in 
other  animals. 

I  The  anomalous  situation  of  these  bones,  and  the  strangely  disguised 
f<jrms  which  their  several  parts  assume,  render  it  veiy  difficult  to  recognise. 


i    CHELONIAN  REPTILES.  327 

humerus  is  remarkably  curved,  especially  in  the  tortoise, 
where  it  has  the  form  nearly  of  a  semi-circle.  The  radius 
and  ulna  are  distinct  from  each  other;  the  carpus  and  pha- 
langes are  short  and  stunted,  forming  a  compressed  kind  of 
hand. 

The  pelvis,  like  the  scapula  and  clavicle,  is  enclosed  with- 
in the  bony  shell  which  protects  the  trunk.  The  sacrum  is 
moveable  upon  the  last  dorsal  vertebra;  and  the  coccygeal 
vertebrae  are  continued  from  it,  forming  a  short  tail.  The 
femur  is  short  and  powerful,  and  somewhat  bent,  but  less  so 
than  the  humerus;  and  the  rest  of  the  bones  of  the  hind  ex- 
tremity are  similar  to  those  of  the  fore  leg.*  All  the  feet 
are  joined  obliquely  to  the  limbs  which  support  them,  giving 
the  animal  an  apparent  awkwardness  of  gait,  as  if  it  were 
obliged  to  walk  upon  club  feet.  The  impulse  which  they 
give  being  lateral  and  oblique,  renders  them  more  efficacious 
for  progression  in  the  water  than  on  land:  this  circumstance, 
in  conjunction  with  the  constitutional  torpor  of  the  animal, 
sufficiently  accounts  for  the  excessive,  and,  indeed,  proverbial 
tardiness  of  its  movements. 

Security  appears  still  to  be  the  object  aimed  at  in  the  me- 
chanism of  all  the  other  parts  of  the  skeleton.  The  articu- 
lations at  the  shoulders  and  the  hips  are  such  as  facilitate 
the  complete  retraction  of  the  limbs  within  the  carapace. 
After  the  head  has  been  drawn  in  by  the  double,  or  serpen- 
tine flexion  of  the  neck,  the  knees  are  brought  together,  and 
the  whole  limb  withdrawn  within  the  shell,  the  fore  legs 
folding  completely  over  the  head,  so  as  to  cover  and  protect 
it  most  effectually.  For  this  purpose,  the  carpus  and  meta- 
carpus are  exceedingly  flattened,  and  approximate  to  the  lin- 

in  the  skeleton,  the  several  pieces  whicli  correspond  to  the  normal  type  of 
the  scapula,  acromion,  coracoid  bone,  and  clavicle;  and  anatomists  arc  not 
yet  ag-recd  as  to  the  proper  designations  which  are  applicable  to  these  bones, 
in  the  Chelonia. 

*  The  cylindrical  bones  of  the  tortoise  are  solid  throug-hout,  and  have  no 
cavity  for  containing-  marrow,  as  in  the  more  hig-hly  developed  bones  of  the 
mammalia.     This  is  seen  iu  the  section  of  the  femur,  Fig'.  214. 


328  THE   MECHANICAL  FUNCTIONS. 

like  form  which  we  shall  presently  see  exemplified  in  the 
cetaceous  tribes.  The  phalanges  are  also  large  and  length- 
ened, forming  a  kind  of  oval  hand,  or  rather  paddle,  the 
functions  of  which  it  is  w^ell  calculated  to  perform.  The 
curvature  of  the  humerus  is  of  great  advantage  to  the  .tor- 
toise in  assisting  it  to  turn  itself,  when,  by  any  accident,  it 
has  been  laid  on  its  back. 

Considerable  differences  may  be  noticed  in  the  structure 
of  the  several  species  of  Chelonia,  according  to  the  diversity 
of  their  habits.  Tortoises  which  live  on  land,  require  more 
complete  protection  by  means  of  their  shell  than  turtles,  or 
Emydes,  which  dwell  only  in  the  water:  hence  the  convex- 
ity of  their  carapace,  the  solidity  of  its  ossification,  its  im- 
moveable connexion  with  the  plastron,  and  the  complete 
shelter  it  affords  to  the  head  and  limbs.  Turtles,  on  the 
other  hand,  receiving  support  from  the  element  in  which 
they  reside,  require  less  provision  to  be  made  for  these  ob- 
jects. Their  carapace  is  smaller,  has  a  more  flattened  form, 
and  cannot  afford  protection  to  the  head  and  limbs.  These 
latter  organs  are  proportionally  larger,  present  a  greater  de- 
velopment of  the  radius  and  ulna,  and  are  compressed  into 
a  flat  expanded  surface.  Previously  to  the  retraction  of  the 
head  and  limbs  within  the  shell,  the  air  is  expelled  from  the 
large  cavities  of  the  lungs,  by  the  vigorous  actions  of  the  ab- 
dominal muscles,  which  exist  in  these  animals  as  well  as  in 
all  the  vertebrata,  although  here  they  are  covered  by  the 
bones,  and  compress  the  lungs  by  pushing  the  abdominal 
viscera  against  them.  This  sudden  expulsion  of  air  is  the 
cause  of  the  long  continued  hissing  sound  which  the  tortoise 
emits  while  preparing  to  retreat  into  its  strong  hold. 

The  ribs,  though  they  first  assume  the  form  of  broad 
plates  immoveably  united  to  the  spine,  when  they  have  pro- 
ceeded a  certain  distance,  separate  from  each  other,  and  re- 
sume their  usual  form;  the  intervening  spaces  between  two 
adjacent  ribs  being  here  filled  up  by  membrane.  The  plas- 
tron is  united  with  the  carapace  by  membrane,  likewise;  and 
the  sternum,  instead  of  forming  one  broad  plate  of  bone,  has 


CHELONIAN  REPTILES.  329 

the  intervals  between  its  imperfectly  developed  elements 
also  membranous.  All  this  renders  the  whole  shell  less 
compact,  more  flexible,  and  more  feeble:  but  the  movements 
of  the  animal  are  quicker  and  more  energetic. 

These  characteristic  differences  between  Ihe  aquatic  Che- 
Ionia  and  those  that  live  on  land  are  still  more  strongly- 
marked  in  the  genus  Trionyx,  or  soft  tortoise;  which  is 
destitute  of  scales,  and  in  which  many  of  the  pieces  that  are 
bony  in  the  tortoise  are  replaced  by  simple  cartilage  or 
membrane. 

The  enormous  weight  of  the  shell  of  the  turtle  would  be 
a  serious  impediment  to  the  motion  of  this  animal  in  the 
water,  were  there  not  some  provision  made  for  diminishing 
the  specific  gravity  in  the  body.  This  purpose  is  answered 
by  the  great  capacity  of  the  lungs,  which,  when  inflated 
with  air,  nearly  fill  the  thorax,  and  give  great  buoyancy  to 
the  whole  mass.  Thus,  wherever  there  exists  a  supposed 
inconvenience,  dependent  on  the  fulfilment  of  one  condi- 
tion, we  are  certain  to  meet  with  a  compensation  in  the 
structure  of  some  other  part,  and  in  the  mode  of  executing 
some  o^her  function.  An  express  provision  for  giving  buoy- 
ancy has  been  made  in  the  construction  of  the  shell  of  a  spe- 
cies of  tortoise  inhabiting  the  coasts  of  the  Scychelle  Islands. 
The  under  surface  of  the  shell,  instead  of  being  gently  con- 
cave, as  in  land  tortoises,  has  a  deep  circular  concavity  in 
the  centre,  above  four  inches  in  depth,  which,  when  the 
animal  goes  into  the  water,  retains  a  large  volume  of  air, 
buoying  up  the  whole  mass  while  it  remains  in  that  ele- 
ment* The  greater  size  of  turtles,  when  compared  with 
tortoises,  is  a  farther  instance  of  the  superior  facility  with 
which  organic  growth  proceeds  in  aquatic  than  in  land  ani- 
mals formed  on  the  same  model  of  construction. 

*  Home's  Lectures,  vi.  37. 


Vol.  I.  42 


(     330     ) 


CHAPTER  IX. 

MAMMALIA. 

§  1.  Mammalia  in  general. 

The  singular  animals,  so  remarkable  for  their  anomalous 
shapes,  their  torpid  vitality,  and  their  amphibious  constitu- 
tion, which  have  lately  occupied  our  attention,  appear  placed 
by  nature  as  forms  of  transition,  in  the  passage  from  those 
vertebrated  animals  which  dwell  in  the  water,  to  those 
which  inhabit  the  land.  The  class  of  Mammifera^  or  Mam- 
malia^  comprehends  all  the  animals  which  possess  a  spinal 
column,  breathe  air  by  means  of  lungs,  and  are  also  warm- 
blooded, and  viviparous,  conditions  which  render  it  neces- 
sary that  they  should  possess  organs,  called  m^ammse,  en- 
dowed with  the  power  of  preparing  milk  for  the  nourish- 
ment of  their  young;  a  peculiarity  from  which  the  lame  of 
the  class  is  derived.  But  they  are  not  exclusively  land 
animals;  for  among  the  mammalia  must  be  ranked  several 
amphibious  and  aquatic  tribes,  such  as  the  seal,  the  walrus, 
the  porpus,  the  dolphin,  the  narwal,  the  cachalot,  and  the 
whale;  animals  which,  however  widely  they  may  differ  in 
their  habits  and  external  conformation  from  terrestrial  quad- 
rupeds, possess,  in  common  with  the  latter,  all  the  essential 
characters  of  internal  structure  and  of  Junctions  above  enu- 
merated. These  characters  belong  also  to  the  human  spe- 
cies, which  must  consequently,  in  its  zoological  relations,  be 
ranked  as  a  genus  of  the  class  mammalia.  So  numerous, 
indeed,  are  the  analogies  wjiich  connect  the  natural  families 
of  this  class  with  our  own  race,  that  we  must  ever  feel  a 
deep  interest  in  the  accurate  investigation  of  their  compara- 
tive anatomy  and  physiology;  and  it  has  been  found,  accord- 
ingly, that  the  progress  which  has,  of  late  years,  been  made 


MAMMALIA.  331 

in  this  branch  of  science  has  materially  enlarged  our  know- 
ledge of  the  structure,  the  functions,  and  the  physical  his- 
tory of  man:  subjects  with  which  our  welfare  has  obviously 
the  closest  and  most  intimate  relation. 

The  principle  of  analogy  which  prevails  so  generally  in 
the  inferior  departments  of  the  animal  creation,  may  be  also 
traced  in  the  class  mammalia;  for  we  always  fmd  its  influ- 
ence more  conspicuous  in  proportion  as  the  objects  compre- 
hended in  the  natural  series  of  beings  are  more  numerous 
and  more  diversified.  Scarcely  any  of  the  great  natural  as- 
semblages of  animals  exhibit  more  variety  in  their  habits 
and  modes  of  existence,  than  the  one  we  are  now  examining. 
Each  race  has. its  peculiar  destination  with  regard  to  the 
kind  of  food  by  which  it  is  nourished,  and  the  means  by 
which  that  food  is  obtained.  The  carnivorous  tribes  wase 
war  with  the  larger  animals,  whom  they  either  spring  upon 
unawares,  or  openly  pursue  and  overpower,  displaying  the 
savage  energies  of  their  nature,  in  practising  all  the  arts  of 
ferocious  and  sanguinary  destruction.  Others,  intent  on 
meaner  prey,  resort  to  divers  stratagems  for  its  possession; 
some  are  designed  to  feed  chiefly  on  the  mollusca,  and  others 
swallow  insects  only.  The  numerous  tribes  which  are 
formed  to  subsist  on  vegetable  food  exhibit,  in  like  manner, 
a  great  diversity  of  constructions,  adapted  to  the  particular 
nature  of  that  subsistence,  whether  it  be  herbage,  or  the 
leaves  of  trees,  or  fruits,  or  seeds,  or  the  coarse  fibres  of  the 
wood  and  bark.  While  all  are  gifted  with  powers  to  ob- 
tain the  nourishment  they  require,  those  that  have  not  been 
armed  with  weapons  of  attack,  are  still  provided  with  in- 
struments of  defence,  or  with  means  of  flight.  Each  has  its 
respective  sphere  of  operation;  and  to  each  has  its  appropri- 
ate soil,  habitation,  climate,  and  element  been  assigned. 

It  is  easy  to  conceive  that  all  these  various  circumstances 
must  lead  to  great  diversities  in  the  apparatus  for  mastica- 
tion and  for  digestion,  in  the  organization  of  the  senses,  in 
the  construction  of  the  instruments  of  locomotion  and  of  pre- 
hension, and  in  the  general  form  of  the  body  to  which  these 


332  THE  MECHANICAL  FUNCTIONS. 

various  parts  are  to  be  adapted.  Yet,  amidst  all  these  varia- 
tions, we  may  perceive  the  same  laws  of  analogy  connecting 
the  whole  into  one  series,  and  assimilating  all  these  multi- 
form structures  to  one  common  standard.  The  same  organ, 
however  modified  in  its  shape  and  size,  however  stinted  in 
one,  or  developed  in  another,  is  ever  found  in  its  appropriate 
place,  and  retains  the  same  connexions  with  adjacent  organs, 
whether  we  seek,  it  in  the  carnivorous  or  the  herbivorous 
quadruped,  in  the  inhabitant  of  the  land  or  of  the  water,  in 
the  denizen  of  the  frigid  or  of  the  torrid  zone;  or  in  animals 
of  the  most  diminutive  or  most  colossal  statures. 

As  an  example,  we  may  take  the  vertebrae  of  the  neck. 
It  is  a  universal  law,  that  this  part  of  the  spinal  column 
shall,  in  every  animal  of  the  class  mammalia,  consist  of 
neither  more  nor  less  than  seven  vertebrae.  Whatever  be 
the  length  or  shortness  of  the  neck,  whether  it  be  compressed 
into  a  small  space,  as  in  the  elephant  and  the  mole,  whether 
it  be  lengthened  to  allow  the  head  to  reach  the  ground,  as 
in  the  horse  and  the  ox,  or  whether  it  be  excessively  pro- 
longed to  allow  the  animal  to  reach  the  tops  of  the  trees,  as 
in  the  cameleopard,  still  this  same  constant  number  is  pre- 
served in  the  vertebrae  which  it  contains.  When  the  neck 
is  long,  each  individual  vertebra  must  necessarily  be  length- 
ened in  the  same  proportion.  Thus,  in  the  Cameleopard, 
the  vertebrae  of  the  neck  consist  of  seven  very  long  tubes, 
joined  together  endwise,  w^ith  scarcely  any  development  of 
spinous  processes,  lest  they  should  impede  the  bending  of 
the  neck.  The  greatest  contrast  to  this  structure  is  met 
with  in  the  Dolphin,  and  other  Cetacea,  which  present  ex- 
ternally no  appearance  whatever  of  a  neck,  but  whose  skeleton 
exhibits  cervical  vertebrae,  closely  compressed  together,  and 
exceedingly  thin,  and  most  of  them  united  together;*  every 
bone,  thus  formed,  however,  retains  the  marks  of  having 
originally  consisted  of  separate  vertebrae;  and  still,  in  this 

*  In  the  cachalot,  the  whole  of  these  seven  vertebrae  are  usually  anchy- 
losed  into  one  bone. 


CETACEA.  333 

extreme  case,  the  number  of  primary  pieces  is  constantly 
seven.* 

§  2.  Cetacea. 

Remarkable  exemplifications  of  the  law  of  uniformity 
of  organic  structure  are  furnished  by  the  family  of  the  Ce- 
tacea, which  includes  the  whale,  the  cachalot,  the  dolphin, 
and  the  porpus,  and  exhibits  the  most  elementary  forms  of 
the  type  of  the  mammalia,  of  which  they  represent  the  earl)^, 
or  rudimental  stage  of  development.  Here,  as  before,  we 
have  to  seek  these  first  elements  among  the  inhabitants  of 
the  vvater:  for  whenever,  in  our  progress  through  the  ani- 
mal kingdom,  we  enter  upon  a  new  division,  aquatic  tribes 
are  always  found  to  compose  the  lowest  links  of  the  ascend- 
ing chain.  Here,  also,  we  observe  organic  development 
proceeding  with  more  rapidity,  and  raising  structures  of 
greater  dimensions  in  aquatic  than  in  terrestrial  animals. 
The  order  Cetacea  comprises  by  far  the  largest  animals 
which  inhabit  the  globe.  Whatever  may  have  been  the 
magnitude  of  those  huge  monsters  which  once  moved  in 

*  The  Bradypus  tridadylus,  or  three-toed  sloth,  was,  till  very  lately, 
thoug-ht  to  constitute  a  notable  exception  to  this  law,  being-  described  as 
having-  nine,  instead  of  seven,  cervical  vertebrae.  It  is  now  found,  however, 
that  the  two  last  of  these  vertebrae,  which  appeared  to  be  supernumerarv, 
ought  properly  to  be  classed  among-  the  dorsal  vertebrae,  of  wiiich  they  possess 
the  distinctive  characters,  not  only  from  the  form  and  size  of  tiieir  transverse 
processes,  but  also  from  their  having-  small  bony  appendices,  articulated  with 
them  by  a  reg-ular  joint  at  their  extremities,  and  corresponding  exactly,  both 
in  shape  and  situation,  to  the  ribs,  of  which  they  may,  in  fact,  be  considered 
as  rudiments.  These  small  bones  have  been  observed,  both  by  Meckel  and 
by  Cuvier,  attached  to  the  ninth  vertebra:  and  Mr.  T.  Bell  has  recently  not 
only  confirmed  the  observations  of  these  anatomists,  but  has  farther  discovered, 
that  similar  rudimental  ribs  are  attached  also  to  the  eighth  vertebra.  (See 
Philosophical  Magazine,  third  series,  iii.  376.)  The  Brudypiis  forquaius, 
which  has  been  said  to  possess  eight  cervical  vertebrae,  will,  perhaps,  on  closer 
examination,  be  hereafter  found  not  to  deviate,  any  more  than  the  three-toed 
sloth,  from  the  normal  type,  as  reg-ards  the  number  of  these  vertebrx.  In- 
stances have  occiu'red  of  supernumerary  cervical  processes,  or  ribs  in  the  hu- 
man skeleton.     (See  Edinburg-li  Medical  and  Surg-icid  Journal,  xl.  304.) 


# 


334  THE  MECHANICAL  FUNCTIONS. 

the  bosom  of  the  primeval  ocean,  or  stalked  with  gigantic 
strides  across  antediluvian  plains,  and  whose  scattered  re- 
mains bear  fearful  testimony  of  the  convulsions  of  a  former 
world,  certain  it  is  that,  at  the  present  day,  the  whales  of 
the  northern  seas  are  the  most  colossal  of  the  living  animal 
structures  existing  on  the  surface  of  this  planet. 

A  cursory  survey  of  the  organization  of  the  tribes  belong- 
ing to  this  semi-amphibious  family,  will  impress  us  with  the 
resemblance  they  bear  to  fishes;  for  they  present  the  same 
oval  outline  of  the  body,  the  same  compact  form  of  the 
trunk,  which  is  united  with  the  head  without  an  intervening 
neck;  the  same  fin-like  shape  of  the  external  instruments  of 
motion,  and  the  same  enormous  expansion  and  prolongation 
of  the  tail,  which  is  here  also,  as  in  fishes,  the  chief  agent  in 
progression.  With  all  this  agreement  in  external  charac- 
ters, their  internal  economy  is  conducted  upon  a  totally  dif- 
ferent plan;  for  although  constantly  inhabiting  the  ocean, 
their  vital  organs  are  so  constructed  as  to  admit  of  their 
breathing  only  the  air  of  the  atmosphere,  and  the  conse- 
quences which  flow  from  this  difference  are  of  great  import- 
ance. The  necessity  of  aerial  respiration  compels  them  to 
rise,  at  short  intervals,  to  the  surface  of  the  water;  and  this 
air,  with  which  they  fill  their  lungs  in  respiration,  gives 
their  bodies  the  buoyant  force  that  is  required  to  facilitate 
their  ascent,  and  supersedes  the  necessity  of  a  swimming 
bladder,  an  organ  which  is  so  useful  to  the  fish. 

With  the  intent  of  diminishing  still  farther  their  specific 
gravity,  nature  has  provided  that  a  large  quantity  of  oily 
fluid  shall  be  collected  under  the  skin,  a  provision  which  an- 
swers, also,  the  purpose  of  preserving  the  vital  warmth  of 
the  body.  A  great  accumulation  of  this  lighter  substance  is 
formed  on  the  upper  part  of  the  head,  apparently  with  a  view 
to  facilitate  the  elevation  to  the  surface  of  the  blowing  hole, 
or  orifice  of  the  nostrils,  which  is  placed  there.* 

Another  peculiarity  of  conformation,  in  which  the  cetacea 

*  The  substance  called  Spermaceti  is  lodg-ed  in  cells,  formed  of  a  cartilagi- 
nous substance,  situated  on  the  upper  part  of  the  head  of  the  Cachalot. 


CETACEA.  335 

differ  from  fishes,  and  which  has  also  an  obvious  relation  to 
their  peculiar  mode  of  breathing,  is  in  the  form  of  the  tail, 
which,  instead  of  being  compressed  laterally,  and  inflected 
from  side  to  side,  as  in  fishes,  is  flattened  horizontally,  and 
strikes  the  water  in  a  vertical  direction,  thereby  giving  the 
body  a  powerful  impulsion,  either  towards  the  surface,  when 
the  animal  is  constrained  to  rise,  or  downwards,  when,  by 
diving,  it  hastens  to  escape  from  danger. 

All  the  essential  and  permanent  parts  of  the  skeleton  of 
vertebrated  animals,  that  is,  the  spinal  column,  and  its  im- 
mediate dependencies,  the  skull,  the  caudal  prolongation, 
and  the  ribs,  are  found  in  that  of  the  Cetacea.  The  thorax 
is  carried  very  much  forwards,  especially  in  the  whale,  and 
the  neck  is  so  short  as  to  be  scarcely  recognisable:  for  the 
object  of  the  conformation  is  here,  as  in  that  of  the  fish,  to 
allow  free  scope  for  the  movements  of  the  tail,  and  ample 
space  for  the  lodgement  of  its  muscles.  For  the  purpose  of 
giving  greater  power  and  more  extensive  attachment  to  these 
muscles,  the  transverse  processes  of  the  dorsal  and  lumbar 
vertebrae  are  expanded  both  in  length  and  breadth,  and,  be- 
ing situated  horizontally,  offer  no  impediment  to  the  vertical 
flexure  of  the  spine.  For  the  same  reason,  the  ribs  are  con- 
tinued in  a  line  with  the  transverse  processes,  and  articu- 
lated with  their  extremities,  thus  giving  still  farther  breadth 
to  the  trunk. 

As  there  is  a  total  absence  of  hinder  extremities,  so  there 
is  no  enlargement  of  any  of  the  vertebrae  corresponding  to  a 
sacrum,  and  the  caudal  vertebrae  are  uninterrupted  continua- 
tions of  those  of  the  trunk.  They  develope,  however,  parts 
which  are  met  with  only  among  fishes  and  reptiles,  namely, 
arches  composed  of  inferior  leaves*  and  spinous  processes, 
enclosing  and  giving  protection  to  a  large  artery.  Although 
the  bones  of  the  legs  do  not  exist,  yet  there  are  found,  in  the 
hinder  and  lower  part  of  the  trunk,  concealed  in  the  flesh, 
and  quite  detached  from  the  spine,  two  small  bones,  appa- 

*  These  leaves  being-  formed  of  cartilag-e,  are  generally  lost  when  tlie 
bones  are  macerated  for  the  purpose  of  preparing-  the  skeleton. 


336 


THE  MECHANICAL  FUNCTIONS. 


rently  corresponding  to  pelvic  bones,  for  the  presence  of 
which  no  more  probable  reason  can  be  assigned  than  the 
tendency  to  preserve  an  analogy  with  the  more  developed 
structures  of  the  same  type. 

A  similar  adherence  to  the  law 
of  uniformity  in  the  plan  of  con- 
struction of  all  the  animals  belong- 
ing to  the  same  class,  is  strikingly 
shown  in  the  conformation  of  the 
bones  of  the  anterior  extremities  of 
the  cetacea;  for,  although  they  pre- 
sent, externally,  no  resemblance  to 
the  leg  and  foot  of  a  quadruped,  being 
fashioned  into  fin-like  members,  wdth 
a  flat,  oval  surface,  for  striking  the  wa- 
ter, yet,  when  the  bones  are  stripped 
of  the  thick  integument  which  covers 
them  and  conceals  their  real  form, 
we  find  them,  (as  may  be  seen  in  Fig. 
216)  exhibiting  the  same  divisions 
into  carpal  and  metacarpal  bones,  and 
phalanges  of  fingers,  as  exist  in  the 
most  highly  developed  organization, 
not  merely  of  a  quadruped,  but  also 
of  a  monkey,  and  even  of  man. 


§  3.  */lmphibia. 


In  the  small  tribe  denominated  by  Cuvier  Amphibia,  and 
consisting  of  the  Phoca,  or  Seal,  and  the  Tricheciis,  or 
Walrus,  we  perceive  that  an  advance  is  made  towards  a 
fuller  development  of  the  limbs:  these  animals  having  a  dis- 
tinct neck  and  pelvis,  and  both  hind  and  fore  extremities. 
In  the  seal  the  hind  legs  are  drawn  out  posteriorly  to  a  con- 
siderable length,  and  placed  parallel  to  each  other:  when 
united  and  alternately  raised  and  depressed,  they  perform 


AMPHIBIA.  337 

the  same  office  as  the  tail  of  the  cetacea,  and  propel  the  ani- 
mal forwards:  but  when  employed  scparatcl}',  they  are  more 
qualified  to  act  as  oars.  The  walrus  has  feet  still  more  de- 
veloped, and  distinctly  divided  into  toes,  which  arc  disposed 
so  as  to  strike  backwards  against  the  water. 


§  4.  Ma 7717711  fcr oil s  Quachnipeds  in  general. 

From  the  imperfectly  developed  a'quatic  and  amphibious 
tribes,  we  gradually  ascend  to  the  more  finished  structures  of 
mammiferous  quadrupeds,  which  are  expressly  fitted  for 
progression  on  land.  In  these  the  powers  of  development, 
not  being  expended  in  the  mere  effort  of  giving  expansion 
to  the  several  textures,  and  of  swelling  the  bulk  of  the  frame, 
sometimes  to  inordinate  dimensions,  are  employed  rather  in 
reducing  the  elements  of  the  organization  into  compact  forms, 
and  in  concentrating  their  energies,  so  as  ultimately  to  at- 
tain the  extent  of  power  and  harmony  of  action,  which  are 
displayed  in  the  higher  orders  of  warm-blooded  quadrupeds. 

It  is  to  these  favoured  tribes  that  we  must  look  for  exam- 
ples of  the  most  complete  development  of  the  skeleton,  and 
the  most  advantageous  disposition  of  mechanic  force.  We 
have  seen  that  reptiles,  from  the  comparative  shortness  of 
their  limbs,  and  the  torpidity  of  their  muscular  powers,  are 
but  ill  adapted  for  rapid  progression.  In  all  the  more  per- 
fectly formed  quadrupeds  of  the  class  mammalia,  the  trunk 
of  the  body,  being  raised  high  upon  the  limbs,  possesses  great 
range  of  motion,  and  can  traverse  with  fewer  steps  a  given 
space. 

The  office  of  the  limbs,  as  far  as  they  are  concerned  in 
progressive  motion,  is  two-fold.  They  have,  first,  to  sus- 
tain the  weight  of  the  body,  which  they  must  do  by  acting 
in  opposition  to  the  force  of  gravity;  and  they  must,  second- 
ly, give  the  body  an  impulse  forwards.  Let  us  consider 
more  particularly  the  relations  which  the  structures  bear  to 
each  of  these  two  functions. 

Vol.  I.  43 


338  THE  MECHANICAL  FUNCTIONS, 

The  limbs  of  quadrupeds  constitute  four  columns  of  sup- 
port to  the  trunk,  which  is  placed  horizontally  above  them; 
but  the  whole  weight  of  the  body,  together  with  that  of  the 
head  and  neck,  does  not  bear  equally  upon  them;  the  fore 
extremities  almost  always  sustain  the  greater-  part  of  that 
weight,  both  because  the  fore  part  of  the  trunk  is  itself  hea- 
vier than  the  hind  part,  and  because  it  is  loaded  with  the 
additional  weight  of  the  head  and  neck.    Hence,  in  the  usual 
attitude  of  standing,  the  pieces  of  which  the  fore  limbs  are 
composed  are  required  to  be  placed  more  in  a  straight  line 
than  those  of  the  hinder  limb:  for  the  power  of  a  column  to 
support  a  weight  is  the  greater  in  proportion  as  it  approaches 
to  the  perpendicular  position.     The  hind  limbs  are  composed 
of  exactly  the  same  number  of  divisions;  but  the  separate 
portions  are  usually  longer  than  those  of  the  fore  extremity, 
and  consequently  if  they  had  been  disposed  vertically  in  a 
straight  line,  they  would  have  elevated  the  hinder  part  of 
the  trunk  to  too  great  a  height  compared  with  the  fore  part. 
This  is  obviated  by  their  forming  alternate  angles  with  one 
another.     As  the  pelvis  connects  the  spine  with  the  joint  of 
the  hip,  and  even  extends  farther  backwards,  the  thigh  bone 
must  necessarily  be  brought  forwards;   then  the   tibia  and 
fibula,  which  compose  the  bones  of  the  leg,  must  be  carried 
backwards  to  their  junction  with  the  bones  of  the  foot;  and, 
again,  the  foot  must  be  turned  forwards  in  its  whole  length 
from  the  heel  to  the  extremities  of  the  toes.     On  comparing 
the  positions  of  the  corresponding  divisions  of  the  anterior 
and   posterior  extremities,   we   observe  that  they  incline, 
when  bent,  in  opposite  directions;  for  in  the  former  we  find, 
in  following  the  series  of  bones  from  the  spine,  that  the  sca- 
pula proceeds  forwards,  the  humerus  backwards;  the  radius 
and  ulna  again  forwards,  and  the  fore  foot  backwards,  posi- 
tions which  are  exactly  the  reverse  of  the  corresponding 
bones  of  the  hind  limb.     (See  Fig.  218,  page  350.) 

The  weight  of  the  body,  in  consequence  of  this  alternate 
direction  of  the  angles  at  the  successive  joints,  must  always 
tend,  while  the  quadruped  is  on  its  legs,  to  bend  each  limb; 


MAMMIFEROUS  QUADRUPEDS.  339 

a  tendency  which  is  required  to  be  counteracted  by  the  ac- 
tions of  the  muscles  whicli  are  situated  on  the  external  side 
of  each  of  those  angles.  These  muscles  are  tlie  extensors  of 
the  joints;  that  is,  the  muscles  whicli  tend  to  bring  their 
parts  into  a  straight  line.  It  is,  in  fict,  by  this  muscular  ac- 
tion, much  more  than  by  simple  rigidity,  that  the  limb  sup- 
ports the  superincumbent  weight  of  the  body.  It  is  evi- 
dent that  greater  muscular  force  is  necessary  for  this  purpose 
when  the  joints  are  bent,  than  when  they  are  already  ex- 
tended; and  the  portions  of  the  fore  legs  being  naturally  in 
this  condition,  require  less  power  than  those  of  the  hinder 
legs  to  retain  them  in  their  proper  relative  positions. 

The  most  complete  instance  of  a  vertical  arrangement  of 
the  bones  of  the  extremities  is  seen  in  the  Elephant;  where, 
in  order  to  sustain  the  enormous  weight  of  the  body,  the 
limbs  are  shaped  into  four  massive  columns,  of  which  the 
several  bones  are  disposed  nearly  in  perpendicular  lines.  By 
this  means,  the  body  is  supported  with  scarcely  any  muscu- 
lar effort,  and  the  attitude  of  standing  is,  in  this  anim.al,  a 
state  of  such  complete  repose,  that  it  often  sleeps  in  that  po- 
sition. '  The  elephant  which  was  kept  some  years  ago  at  the 
Menagerie  at  Paris,  although  much  enfeebled  by  a  lingering 
disorder,  was  never  seen  to  lie  down  till  the  day  on  which 
he  died.  When  he  was  in  the  last  stage  of  debility,  what 
seemed  to  give  him  most  distress  was  the  effort  requisite  to 
support  his  head:  and,  in  order  to  relieve  the  muscles  of  the 
neck,  which  were  strained  in  that  exertion,  he  was  in  the 
habit  of  extending  his  trunk  perpendicularly  to  the  ground, 
by  contracting  all  the  muscular  fibres  which  run  transverse- 
ly in  that  organ,  and  thus  formed  a  vertical  prop  for  the 
head.  But  in  almost  all  other  quadrupeds,  the  mere  act  qf 
standing,  though  a  state  of  comparative  rest,  implies,  for  the 
reasons  already  given,  a  degree  of  muscular  exertion,  and 
they  can  enjoy  complete  repose  only  by  letting  the  body  re- 
cline upon  the  ground. 

The  conformation  of  the  hind  extremities,  which,  as  we 
have  seen,  is  not  so  well  calculated  for  the  simple  support  ol 


340  THE  MECHANICAL  FUNCTIONS. 

the  trunk,  is,  on  the  other  hand,  better  adapted  to  give  it 
those  impulses  which  are  to  effect  its  progressive  movements. 
The  nature  of  those  movements,  and  the  order  in  Which  they 
succeed  each  other,  are  different  according  to  the  peculiar 
mode  of  progression  which  the  animal  practises,  the  degree 
of  speed  it  is  desirous  of  exerting,  and  the  particular  end 
it  has  in  view.  The  paces  of  a  quadruped  usually  distin- 
guished, are  the  walk,  the  trot,  the  gallop,  the  amble,  and  the 
bound. 

In  slow  walking,  only  one  foot  is  raised  from  the  ground 
at  the  same  moment,  so  that  three  points  of  support  always 
exist  for  sustaining  the  weight  of  the  body.  If  the  centre 
of  gravity  be  situated,  as  it  generally  is,  nearly  over  the  mid- 
dle of  the  quadrangular  base  formed  by  the  feet,  while  they 
rest  upon  the  ground,  the  first  effort  to  advance  which  the 
quadruped  makes,  propels  the  centre  of  gravity  forwards. 
This  it  accomplishes  by  pressing  one  of  its  hind  legs  against 
the  ground,  which  leg,  being  thus  fixed  by  the  resistance  it 
there  meets  with,  becomes  the  fulcrum  of  the  first  move- 
ments. The  extensor  muscles  of  the  limb  are  now  exerted 
in  giving  the  body  an  impulse  forwards.  As  soon  as  this 
impulse  has  been  given,  the  muscles  which  had  been  in  ac- 
tion are  relaxed,  and  the  leg  is  raised  from  the  ground, 
brought  forwards,  and  laid  down  close  to  the  fore  foot  of  the 
same  side.  This  fore  foot  is  next  raised  and  advanced,  and 
then  the  same  succession  of  actions  takes  place  with  the 
hind  and  the  fore  foot  of  the  other  side. 

An  attentive  examination  of  the  conditions  of  these  suc- 
cessive positions  will  show  that,  amidst  all  the  changes  which 
take  place  in  the  points  of  support,  the  stabilit}^  of  the  body 
is  constantly  preserved.  It  is  an  elementary  proposition  in 
mechanics  that  all  that  is  necessary  for  ensuring  the  sup- 
port of  a  body  on  any  given  base,  is  that  the  vertical  line 
drawn  from  the  centre  of  gravity  shall  fall  within  that  base. 
When  the  animal  is  standing,  the  feet  form  a  quadrilateral 
base,  and  the  centre  of  gravity  is  in  a  vertical  line  passing 
either  through  the  centre  of  the  base,  or,  as,  for  the  reasons 


PROGRESSIVE  MOTION  IN  QUADRUPEDS.  341 

already  mentioned,  more  frequently  happens,  through  a  point 
a  little  in  front  of  the  exact  centre.  At  the  time  when  the 
hind  foot  which  hegan  the  action  is  raised  from  the  ground, 
the  centre  of  gravity,  having  heen  by  that  action  impelled 
forwards,  still  remains  above  the  base  formed  by  the  other 
three  feet,  and  wdiich  is  now  reduced  to  a  trian^i-lc.  That 
hind  foot  being  set  down,  while  the  corre?pondino-  fore  foot 
is  raised,  a  new  triangular  base  is  formed  by  the  same  hind 
foot,  together  witli  the  tw^o  of  the  other  side,  which  have 
not  yet  been  raised.  The  centre  of  gravity  is  still  situated 
above  this  triangle,  and  the  body  is  consequently  still  sup- 
ported on  these  three  feet.  The  fore  foot  may  now  be  ad- 
vanced without  endangering  the  stability  of  the  body:  and 
by  the  time  this  foot  is  set  down,  and  has  thereby  formed  a 
new  quadrilateral  basis  with  the  other  feet,  the  centre  of  o-ra- 
vity  has  arrived  above  the  centre  of  this  new  base.  But  at 
this  moment  the  centre  of  gravity  is  ao;ain  urn-ed  for- 
wards  by  the  other  hind  foot,  which  now  comes  into  action 
and  repeats  on  the  other  side  the  same  succession  of  actions 
which  are  attended  with  the  same  consequences  as  before. 
Thus,  during  its  whole  progress,  the  animal  is  never  for  an 
instant  in  danger  of  falling;  for  whichever  of  the  feet  may  be  ^ 
raised  from  the  ground,  the  other  three  feet  are  always  so 
placed  as  to  form  a  stable  base  of  support. 

In  quick  walking  it  often  happens  that  quadrupeds  raise 
their  fore  foot  on  either  side  a  little  before  the  hind  foot 
comes  to  the  ground.  This  is  shown  by  the  impression 
made  by  the  latter  being  on  the  same  spot,  or  even  rather 
in  advance  of  the  impression  made  by  the  former.  But 
the  time  during  which  the  body  is  thus  supported  only  by 
two  feet  is  so  short  as  not  sensibly  to  influence  the  results. 

In  consequence  of  the  obliquity  of  the  alternate  impulses 
given  to  the  centre  of  gravity  by  the  successive  actions  of 
both  the  hind  legs,  a  slight  degree  of  undulation  is  occa- 
sioned; but  these  undulations  are  only  lateral.  A  trot  may 
be  considered  as  a  succession  of  short  leaps  made  by  each 
set  of  feet  taken  diagonally;  that  is,  by  the  right  fore  foot, 
and  the  left  hind  foot;  or,  vice  versa,  the  one  set  being  raised 


342  THE  MECHANICAL  FUNCTIONS. 

too-ether  a  short  time  before  the  others  have  reached  the 
ground:  so  that  during  that  minute  interval  of  time  all  the 
feet  are  in  the  air  at  the  same  moment;  and  during  the  re- 
maining portion  of  the  time,  the  body  is  resting  upon  the 
two  feet  placed  diagonally  with  regard  to  each  other.  The 
undulations  are  here  chiefly  vertical,  instead  of  lateral,  as 
they  are  in  the  walking  pace. 

A  gallop  is  a  continued  succession  of  longer  leaps  made 
by  the  two  hind  feet  in  conjunction.  In  this  case,  the  cen- 
tre of  gravity  is  lifted  higher  from  the  ground,  and  is  pro- 
jected in  a  wide  arch,  and  with  great  velojcity. 

In  the  amble,  both  the  legs  on  one  side  are  raised  toge- 
ther; so  that  the  impulsions  given  are  directed  much  more 
laterally  than  in  any  other  pace,  and  the  body  is  thrown  into 
a  strong  undulatory  motion  from  side  to  side: 

Another  kind  of  pace  is  the  bound,  which  is  often  prac- 
tised by  deer,  and  is  performed  by  striking  the  ground  with 
all  the  legs  at  the  same  moment.  It  consists,  therefore,  like 
the  gallop,  of  a  series  of  leaps;  but  their  direction  is  more 
uniformly  upwards,  from  the  concurrence  of  all  the  legs  in 
the  same  action. 

Nature  has  purposely  endowed  different  tribes  with  very 
different  capacities  to  execute  progressive  movements,  by 
the  variations  she  has  introduced  into  the  comparative 
lengths  of  the  several  parts  of  the  trunk,  and  the  size  and 
mobility  of  the  extremities.  Of  all  the  large  animals,  the 
Lion  has  been  constructed  with  the  finest  proportions  for 
conferring  both  strength  and  activity.  The  mass  of  his 
body  is  supported  more  by  the  fore  than  by  the  hind  ex- 
tremities. In  walking,  the  lion  takes  long  strides,  and  ex- 
hibits strongly  the  lateral  undulations  of  the  trunk. 

Quadrupeds  having  a  very  long,  or  a  very  massive  body, 
or  whose  limbs  are  short,  and  nearly  of  equal  height,  are  in- 
capable of  advancing  by  a  gallop,  or  at  least  cannot  sustain 
this  pace  without  a  painful  effort,  and  never  but  for  a  short 
time.  The  Tiger,  which  has  a  longer  body  than  the  lion, 
gallops  with  less  facility;  and  runs  chiefly  by  an  accelera- 
tion of  its  walking  pace.     It  excels  principally  in  the  vigour 


PROGRESSIVE  MOTION  IN  QUADRUPEDS.  343 

and  extent  of  its  bounds;  for  which  it  is  admirably  qualified 
by  prodigious  power  of  its  muscles,  enabling  it  to  spring 
forwards  upon  its  victim  witii  an  impetus  which  nothing 
can  resist. 

The  speed  with  which  a  quadruped  is  capable  of  advancing 
depends  more  on  the  disposition  of  the  muscles  and  the  ex- 
tent of  the  articulations,  and  more  especially  on  the  power 
of  the  extensors  of  the  hind  extremities,  than  on  the  form 
of  the  body.  Great  length  and  muscularity  in  the  hind  legs 
are  generally  attended  with  considerable  power  of  leaping. 
This  is  exemplified  in  the  Jerboa  and  the  Kanf;nroo^  ani- 
mals, which,  from  the  disproportionate  shortness  of  their 
fore  legs,  are  totally  incapacitated  from  walking;  and  for 
the  same  reason,  they  cannot  run  with  any  degree  of  swift- 
ness. It  is  only  in  climbing  up  a  steep  acclivity  that  the 
jerboa  is  enabled  to  employ  all  its  limbs:  in  a  descent,  on 
the  contrary,  it  uses  only  its  fore  legs,  the  hinder  being 
dragged  after  them.  But,  when  pursued,  these  animals  are 
capable,  for  a  long  continuance,  of  taking  leaps  of  nine  feet 
distance,  and  of  repeating  these  leaps  so  quickly,  that  the 
Cossacks,  though  mounted  on  the  swiftest  horses,  are  unable 
to  overtake  them. 

The  Kanguroo,  in  almost  all  his  movements,  brings  into 
action  his  powerful  tail,  which  is  furnished  with  very  strong 
muscles,  and  maybe  considered  as  constituting  a  fifth  limb. 
It  is  of  great  assistance  to  the  animal  in  taking  leaps,  and 
during  its  repose,  contributes,  together  with  the  hind  feet, 
to  support  the  weight  of  the  body,  as  on  a  tripod,  and  to 
leave  at  liberty  the  fore  legs,  which  may  then  be  employed 
as  arms. 

The  Hare  and  the  Babbit  furnish  other  instances  of  an 
extraordinary  length  of  the  hinder  legs  depriving  the  ani- 
mal of  the  power  of  walking,  and  obliging  it  to  move  for- 
wards only  by  a  succession  of  leaps.  The  hare  may  be  said, 
indeed,  to  walk  with  its  fore  legs  only,  while  it  gallops  with 
the  hinder:  but  this  disadvantage  is  amply  compensated  by 
its  amazing  swiftness  when  runi;^ing  at  full  speed. 


344  THE  MECHANICAL  TUNCTIONS. 

Animals,  like  the  hare,  in  which,  from  the  great  length 
of  the  hinder  limbs,  the  posterior  half  of  the  body  is  higher 
than  the  anterior,  run  much  better  up  a  declivity  than  on 
level  ground.  In  a  descent,  on  the  contrary,  they  are 
obliged  to  pursue  an  oblique  and  zig-zag  course,  otherwise 
they  would  be  in  danger  of  oversetting,  as  happens  occa- 
sionally to  the  Agouti  and  the  Guinea  pig^  when  these  ani- 
mals attempt  to  run  down  hill. 

The  Sloth,  which  is  formed  for  clinging  with  great  tena- 
city to  the  boughs  of  trees,  presents  a  remarkable  contrast 
to  the  animals  we  have  just  noticed;  its  fore  legs  being  much 
longer  than  the  hinder,  and  its  movements  being  proverbi- 
ally slow.  The  peculiar  modifications  of  its  muscular  powers 
are  probably  consequences  of  the  singular  mode  in  which, 
as  I  shall  afterwards  have  occasion  to  notice,  its  arteries  are 
distributed. 

The  Camcleopard^  likewise,  has  the  fore  legs  much  longer 
than  the  hinder.  The  object  of  this  conformation  was  pro- 
bably to  elevate  the  anterior  part  of  the  spine,  so  as  to  raise 
the  head  as  much  as  possible,  and  also  to  give  a  considera- 
ble inclination  to  the  whole  column,  for  the  purpose  of  dis- 
tributing more  equally  the  weight  of  the  head  and  of  the 
very  long  neck  upon  all  the  legs;  for  the  length  of  the  neck 
is  fully  equal  to  that  of  the  trunk.  It  is  evident  that  if  the 
body  had  been  placed  in  the  usual  horizontal  position,  the 
anterior  extremities  would  have  had  to  support  the  whole 
of  the  enormous  weight  of  this  neck  and  head.  This  pecu- 
liarity of  structure,  however,  introduces  considerable  modi- 
fications in  the  mode  of  progression  of  the  animal.  The 
ordinary  pace  of  the  cameleopard  is  the  amble;  but  it  has 
also  a  slower  walking  pace,  and  occasionally  a  gallop.  In 
the  amble,  its  undulation  is  so  considerable  as  to  give  it  the 
appearance  of  being  lame.  A  similar  kind  of  limping  gait, 
arising  from  the  same  cause,  namely,  the  disproportionate 
elevation  of  the  fore  part  of  the  spine,  has  been  observed  in 
the  Hyena, 


RUMINANT  QUADRUPEDS.  34/5 


§  5.  Buminantia. 

In  following  the  series  of  Mammalia  in  the  order  which 
best  exhibits  their  successive  stages  of  development,  I  shall 
commence  with  those  whose  digestive  apparatus  is  formed 
to  extract  nourishment  exclusively  from  the  vegetable  kino-, 
dom.     The  first  assemblage  that  presents  itself  to  our  notic'^e 
is  the  remarkable  hmWy  oi  Ru77iinants,  which  feed  princi- 
pally on  herbage.     Wherever  the  earth  is  clothed  with  ve- 
getation, it  requires  neither  skill  nor  exertion  on  their  part 
to  seek  and  to  devour  the  rich  repast  which  is  profusely 
spread  under  their  feet.     To  remove  from  one  pasture  to 
another,  to  browse,  and  to  repose,  constitute  the  peaceful 
employments  of  their  lives,  and  satisfy  the  chief  conditions 
of  their  existence.     To  these  purposes  the  whole  conforma- 
tion of  their  skeleton,  and  especially  of  those  parts  which 
constitute  the  limbs,  is  adapted.     The  anterior  extremities 
having  only  to  support  the  weight  of  the  fore  part  of  the 
trunk,  and  to  assist  in  progressive  motion,  have  a  less  com- 
plicated arrangement  of  joints,  and  exhibit  many  of  those 
consolidations  of  the   bones,   which   tend   to   simplify  the 
structure,  and  to  contribute  to  its  strength. 

But  though  never  incited  by  the  calls  of  appetite  to  en- 
gage in  sanguinary  warfare,  they  are  yet  liable  to  the  as- 
saults of  many  ferocious  and  well  armed  adversaries,  and 
often  unprovided  with  any  adequate  means  of  defence;  their 
only  resource,  therefore,  is  to  avoid  the  dangers  of  the  en- 
counter by  a  rapid  and  precipitate  flight.     To  confer  this 
power  appears  to  have  been  the  object  aimed  at  by  nature 
in  every  part  of  the  conformation  of  these  animals.     It  is 
among  the  ruminant  tribes  that  the  fleetest  of  quadrupeds 
are  to  be  found,  such  as  the  gazelle,  the  antelope,  and  the 
deer,  animals  which  exhibit  the  highest  perfection  of  struc- 
ture belonging  to  this  type.     We  may  observe  that  the 
parts  composing  the  hind  legs  are  longer,  and  inclined  to 
Vol.  I.  44 


346 


THE  MECHANICAL  FUNCTIONS. 


one  another  at  angles  more  acute  in  these  animals  than  in 
other  tribes  of  mammalia,  so  that  they  are  always  ready  for 
instantly  commencing  their  flight,  and  springing  forwards 
on  the  slightest  notice  of  danger.    (See  Fig.  218,  page  350.) 
As  it  was  necessary,  from  the  situation  of  their  food,  that 
their  heads  should  reach  the  ground  in  grazing,  w^e  find  that 
the  neck  has  been  much  elongated,  that  the  muscles  which 
raise  the  head  have  been  enlarged  and  strengthened,  and  that 
the  spinous  processes  of  the  back  and  neck  have  been  much 
expanded,  in  order  to  allow  of  suiFicient  surface  for  the  at- 
tachments of  these  muscles.     The  effort  requisite  to  raise, 
and  even  support  the  head,  is  very  considerable;  as  will  ap- 
pear when  we  reflect  that  its  weight  acts  by  means  of  an  ex- 
tremely long  lever;  for  such  is  the  mechanical  office  of  the 
elongated  neck.     But,  in  order  to  economize  the  muscular 
power,  an  elastic  ligament  is  employed  to  sustain  the  weight 
of  the  head.     This,  which  is  termed  the  Ugamentum  niichse, 
and  is  represented  at  n,  in  Fig.  217,  is  formed  of  a  great 
number  of  bands  wiiich  connect  the  hinder  part  of  the  cra- 
nium, at  the  ridge  of  the  occipital  bone,  and  all  the  spinous 
processes  of  the  neck,  with  those  of  the  back,  the  separate 
slips  from  each  being  successively  joined  together,  and  com- 
posing a  ligament  of  great  length  and  power.     It  differs,  in 


its  structure,  from  ordinary  ligaments,  being  highly  elastic, 
so  that  it  yields  to  the  extension  of  the  neck  when  the  animal 


RUMINANT  QUADRUPEDS.  317 

lowers  its  head,  and  gives  considerable  assistance  to  the  mus- 
cles in  raising  it.  In  the  deer  and  the  ox,  which  toss  tiieir 
heads  with  force,  and  especially  in  the  males,  which  are 
armed  with  antlers  or  horns,  the  muscles  performing  those 
motions  are  remarkably  strong,  and  the  spinous  processes  of 
the  back  particularly  prominent.  In  the  loins,  on  the  con- 
trary, we  find  the  transverse  processes  more  enlarged,  for 
the  purpose  of  giving  a  powerful  mechanical  purchase  to  the 
muscles  which  are  inserted  into  them. 

The  chest  of  ruminant  quadrupeds  is  compressed  laterally, 
in  order  to  allow  room  for  the  unrestrained  motions  of  the 
anterior  extremity;  and  the  sternum  projects  so  as  to  resem- 
ble the  keel  of  a  ship.  The  bones  of  the  anterior  extremity 
are  not  joined  to  the  rest  of  the  skeleton  by  means  of  any 
bone  corresponding  to  a  clavicle;  but  they  are  connected 
with  the  spine  and  ribs  only  by  ligaments  and  muscles;  so 
that  the  fore  part  of  the  trunk  is,  in  fact,  suspended  between 
the  limbs  by  its  muscular  attachments  alone.  This  is  not 
the  case  with  the  hind  extremities;  for  their  bones  commence 
with  the  pelvis,  which  proceeds  backwards  from  the  sacrum, 
but  with  a  considerable  inclination  downwards,  and  has  a 
deep  hemispherical  cavity  for  the  lodgement  of  the  round 
head  of  the  thigh  bone.  The  lengthened  forms  of  the  iliac 
bones,  and,  also,  of  the  scapula,  provide  for  the  application 
of  muscles  of  considerable  length,  which  are,  consequently, 
capable  of  communicating  to  the  parts  they  move  a  greater 
veloc^y  than  could  4iave  been  effected  by  muscles  of  equal 
strength,  but  witl:^horter  fibres. 

Both  the  humerus  in  front,  and  the  femur  behind,  are  so 
short  as  to  appear,  on  a  superficial  view,  to  form  part  of  the 
trunk,  being  entirely  enveloped  and  concealed  by  the  large 
muscles  connecting  them  with  the  body.  The  heads  of  the 
two  humeri,  in  consequence  of  the  absence  of  the  clavicle, 
are  brought  very  near  each  other,  so  as  to  occupy  a  situation 
as  nearly  as  possible  underneath  the  weight  which  the  limb 
has  to  support. 

The  radius  and  ulna,  which  are  the  two  bones  of  the  fore 


, 


348  THE  MECHANICAL   FUNCTIONS. 

arm,  although  completely  separate  at  an  early  period  of 
growth,  soon  unite  to  form  but  one  bone.  This  union  be- 
gins at  their  lower  end,  and  proceeds  upwards  to  within  a 
short  distance  from  the  top,  where  a  separation  may  still  be 
observed  in  the  processes  which  project  from  that  end,  form- 
ing for  some  way  down  a  distinct  suture.  This  union  of 
the  two  bones  must,  of  course,  preclude  all  rotatory  motion; 
but  it  is  calculated  to  give  the  joint  great  security :  and  this  ap- 
pears to  have  been  the  main  object  in  the  conformation  of  the 
whole  limb.  The  same  process  of  consolidation  takes  place 
in  the  hind  leg,  between  the  tibia  and  the  fibula,  which  are 
so  completely  united,  as  to  afibrd  scarcely  any  trace  of  their 
having  been  originally  separate. 

The  carpus  and  the  tarsus  are  both  of  very  limited  extent, 
and  consist  of  a  smaller  number  of  pieces  than  usually  oc- 
cur in  these  joints.     The  consolidation  of  parts  is  most  con- 
spicuous in  the  succeeding  division   of  the  limb,  namely, 
that  constituting  the  metacarpus  in  the   anterior,  and   the 
metatarsus  in  the  hind  extremity.     In  either  case  we  find  it 
consisting  not  of  five  bones,  as  in  the  more  highly  organized 
carnivorous  mammalia,  but  of  a  single  bone  only,  termed 
the  cannon  bone.     In  the  early  periods  of  ossification,  how- 
ever, they  each  consisted  of  two  slender  bones,  lying  close 
and  parallel  to  each  other;  but  afterwards  united  by  an  ossi- 
fic  deposition,  which  fills  up  the  interval  between  them, 
and  leaves  behind  no  trace  of  suture.*     In  proportion  as  the 
young  animal  acquires  strength,  the  uni^n  of  these  twcfbones 
becomes  still  more  intimate  by  the  absodtttion  of  the  parti- 
tion which  separated  their  cavities;  so  that  ultimately  they 
constitute  but  one   cylinder,  with   a  single  central  cavity, 
which  is  occupied  by  marrow. 

The  cannon  bone  is  much  elongated,  both  in  the  fore  and 
hind  extremity;  so  that  the  carpus  and  tarsus,  which  are  the 
commencements  of  the  real  feet,  are  raised  considerably 

*  The  observations  which  establish  this  fact  are  detailed  by  G.  St.  Hilaire, 
in  a  paper  in  the  "  Memoires  du  Museum,"  x.  173. 


RUMINANT  QUADRUPEDS.  349 

above  the  ground.  It  is  a  common  mistake,  arising  from 
the  height  of  these  joints,  and  the  names  they  bear  in  ordi- 
nary language,  to  consider  them  as  the  knees  of  the  animal. 
The  slightest  inspection  of  the  skeleton  will  be  suflicient  to 
show  that  what  is  called  the  knee  in  the  fore  leg  is  properly 
the  wrist;  and  in  the  hind  leg,  the  part  so  misnamed  is  re- 
ally the  heel.  Thus,  the  foot,  especially  in  the  posterior  ex- 
tremity, is  of  great  length;  a  structure  which  is  evidently 
intended  to  give  greater  velocity  to  the  actions  of  the  mus- 
cles, while  it  at  the  same  time  ensures  the  utmost  steadiness 
and  security  of  motion. 

At  the  lower  extremity  of  the  cannon  bone  there  are  two 
articular  surfaces,  indicating  the  originally  separate  ends  of 
its  two  component  bones.  They  are  for  the  articulation  of 
the  two  following  bones,  which  are  also  very  long,  and  which 
correspond  in  situation  to  the  first  phalanges  of  the  fino-ers 
and  toes.  These  are  followed  by  a  second  and  third  set  of 
phalanges ;  the  last  of  which  terminate  in  hoofs.  All  rumi- 
nant quadrupeds  have  thus  a  double  hoof;  a  character  which 
is  peculiar  to  this  family. 

Thus,  then,  has  Nature  moulded  the  organs  of  progressive 
motion  in  this^-emarkable  tribe  of  animals  to  accommodate 
them  to  the  peculiar  conditions  of  their  existence,  while  she 
has  still  preserved  their  relations  to  the  primitive  type  of 
the  class  to  which  they  belong.  Thus  has  she  bestowed 
upon  them  the  slender  and  elegant  forms,  so  pleasing  to  the 
eye,  which  characterize  the  fleetest  racer,  and  has  provided 
for  the  agile,  yet  firm  and  secure  movements  which  they  are 
to  exercise  in  various  ways  in  eluding  the  observation,  and 
escaping  from  the  pursuit  of  their  stronger  and  more  saga- 
cious foes.  This  purpose  they  efiect,  at  one  time  by  rapid 
flight  across  extensive  tracts  of  country;  at  another,  by  re- 
tirement into  unfrequented  forests,  or  mountains  of  difficult 
access,  crossing  their  rugged  surfaces  in  all  directions,  clam- 
bering their  precipitous  acclivities,  and  fearlessly  bounding 
over  intervening  abysses,  from  point  to  point,  till  the  place 
of  safety  is  attained  on  some  rocky  eminence.     From  this 


350 


THE  MECHANICAL  FUNCTIONS. 


secure  station  the  Alpine  chamois  looks  down  upon  its  pur- 
suers, and  defies  their  farther  efforts  at  capture  or  molestation. 


The  astonishing  feats  of  agility  practised  by  this  animal,  and 
by  which  the  most  experienced  hunters  are  perpetually  baf- 


RUMINANT  QUADRUPEDS.  351 

fled  in  their  attempts  to  approach  it,  sufliciently  attest  the 
perfection  of  its  orj^anization  in  reference  to  all  these  ob- 
jects. The  chamois  has  often  been  seen  to  leap  down  a  per- 
pendicular precipice  of  twenty  or  thirty  feet  in  hei<rht, 
without  sustaining  the  slightest  injury.  How  the  ligaments 
that  bind  the  joints  can  resist  the  violent  strains  and  concus- 
sions they  must  be  exposed  to  in  these  quick  and  jarring 
efforts,  is  truly  wonderful. 

While  Nature  has  provided  these  animals  with  the  means 
of  safety  from  their  more  formidable  enemies,  she  has  not 
left  them  altogether  without  defence  against  their  more  equal 
rivals  in  the  field.  It  is  on  the  head  that  she  has  implanted 
those  powerful  arms  which  are  sometimes  wielded  with  dead- 
ly effect  in  their  mutual  combats.  Even  when  not  furnished 
with  horns,  the  animal  instinctively  strikes  with  its  fore- 
head where  the  frontal  bone  has  been  expanded  and  forti- 
fied apparently  with  a  view  to  this  mode  of  attack.  Thus, 
the  ram  butts  with  its  head  without  reference  to  the  horns, 
which  are  coiled  so  as  to  be  turned  away  from  the  object  to 
be  struck.  In  the  deer  and  the  ox  tribes,  however,  the  horns 
are  formidable  weapons  of  offence:  and  it  will  be  interest- 
ing to  inquire**into  the  nature  of  these- organs,  and  the  phe- 
nomena of  their  production. 

The  antlers  of  the  male  stag  are  osseous  structures,  sup- 
ported on  short  and  solid  tubercles  of  the  frontal  bone:  af- 
ter remaining  nearly  a  year  they  are  cast  off,  and  soon  re- 
placed by  a  newly  formed  antler,  which  is  of  larger  size  than 
the  one  which  was  lost.  Previously  to  the  formation  of 
this  structure,  those  branches  of  the  artery,  termed  the  ca- 
rotid,  which  supply  blood  to  the  frontal  bone,  are  observed 
very  rapidly  to  dilate,  and  to  throb  with  unusual  force;  and 
all  the  blood  vessels  of  the  skin  of  the  part  where  the  antler 
is  to  arise,  soon  become  distended  with  blood;  an  effect 
which  is  accompanied  by  general  heat  and  redness,  like  a 
part  in  a  state  of  high  inflammation.*     Presently  the  skin  is 

*  These  phenomena  are  connected  with  periodical  changes  in  the  consti- 
tution relating  to  the  productive  functions. 


352  THE  MECHANICAL  FUNCTIONS. 

elevated  by  the  growth  of  a  tubercle  from  the  subjacent 
bone:  this  tubercle  is  at  first  a  cartilage,  and  after  it  has  at- 
tained a  certain  size,  becomes  ossified,  and  grows  like  other 
osseous  structures,  first  shooting  into  the  form  of  a  length- 
ened cylinder,  and  then  dividing  into  branches.  It  is  fol- 
lowed in  its  elongation  by  the  skin,  which  during  the  whole 
time  that  the  antler  is  growing  is  extended  over  it  in  every 
part,  forming  what  is  called,  from  the  delicate  investment  of 
hair,  its  velvet  coat.  The  blood  vessels  of  the  proper  mem- 
brane of  the  antler,  or  periosteum,  still  continuing  to  sup- 
ply it  with  the  materials  required  for  its  growth  and  conso- 
lidation, deposite  so  great  an  abundance  of  bony  matter,  that 
its  enlargement  is  exceedingly  rapid.  The  whole  antler, 
which  often  weighs  nearly  thirty  pounds,  has  been  known 
to  be  completely  formed  in  ten  .weeks  from  the  time  of  its 
first  appearance.  There  is  no  other  instance  in  the  animal 
kingdom  of  so  rapid  a  growth;  which  is  the  more  remarka- 
able  from  its  occurring  in  a  small  part  of  the  system,  and 
in  a  bony  structure. 

After  the  antler  has  attained  its  full  size,  a  deposition  of 
osseous  substance  still  continuous  at  its  base,  around  the 
trunks  of  the  arteries  which  are  proceeding  along  the  invest- 
ing membrane  of  the  bone  for  the  purpose  of  conveying 
nourishment.  The  accumulation  of  this  substance  raises  a 
ring  called  the  burr,  round  that  part  of  the  antler:  and  by 
encroaching  on  the  arteries  themselves,  it  gradually  dimi- 
nishes their  capacity  of  conveying  blood,  and  they  at  length 
become  entirely  obliterated.  The  bone,  no  longer  receiving 
a  superabundant  nourishment,  ceases  to  grow;  the  integu- 
ments which  covered  it,  decay,  and  becoming  dry  and  shri- 
velled, are  torn  by  rubbing  against  trees,  and  peel  o^  in  long 
shreds,  leaving  the  antler  exposed,  which,  by  the  continued 
effects  of  the  same  kind  of  friction,  soon  acquires  a  polished 
surface. 

During  many  months,  the  antler  being  sufficiently  nourished 
by  its  own  interior  vessels,  continues  in  a  living  state,  and 
preserves  its  connexion  with  the  system.     But,  at  length,  the 


RUMINANT  QUADRUPEDS.  353 

arteries,  whether  from  the  effect  of  the  progressive  deposition 
of  osseous  matter,  or  from  some  change  in  the  balance  of  the 
vital  powers,  shrink,  and  become,  by  degrees,  obliterated. 
The  antler  dies  in  consequence,  and,  although  it  continues  to 
adhere  to  the  skull,  it  is  only  as  a  foreign  body,  and  it  is  not 
long  destined  to  remain  thus  attached ;  for  the  absorbent  ves- 
sels are  now  actively  employed  in  scooping  out  a  groove  of 
separation  between  the  living  and  the  decayed  substance,  at 
the  place  where  the  base  of  the  antler  is  contiguous  to  the 
frontal  bone.     As  soon  as  this  has  proceeded  to  a  sufficient 
depth,  the  adhesion  ceases,  and  the  slightest  concussion  occa- 
sions the  fall  of  the  whole  structure.     After  the  separation  of 
the  antler,  the  eminence  of  the  frontal  bone,  on  which  it  stood, 
is  left  rough  and  uneven,  like  that  of  a  fractured  part :  but 
the  surrounding  integuments  soon  close  over,  and  cover  it 
completely ;  until  the  period  arrives  when  it  is  to  be  replaced 
by  a  new  antler,  which  exhibits  the  same  succession  of  phe- 
nomena, in  its  growth  and  decay,  as  its  predecessor,  only  that 
its  development  is  usually  carried  farther,  the  new  stem  being 
both  thicker  and  longer,  and  the  branches  wider  and  more 
numerous.     The  antler  of  each  successive  year  has,  conse- 
quently, a  different  form  from  that  of  the  preceding;  and, 
when  the  animal  has  attained  a  certain  age,  the  extremities 
of  the  branches  present  broad  expansions  of  bone,  which  the 
antlers  of  an  earlier  growth  had  neveV  exhibited. 

The  short  bony  processes  which  extend  in  a  perpendicular 
direction  on  the  head  of  the  cameleopard,  are  analogous,  in 
some  of  the  circumstances  of  their  formation,  to  the  antlers  of 
the  deer,  being  of  an  osseous  nature,  and  continuous  with  the 
frontal  bone :  but,  in  other  respects,  they  are  very  different ; 
for,  instead  of  being  annually  shed,  they  remain  through  life, 
and  continue  to  be  covered  with  the  integuments,  which  re- 
tain, at  the  extremities,  a  tuft  of  hair.  The  development  of 
these  processes,  in  the  young  animal,  takes  place  in  the  same 
manner  as  that  of  an  antler,  but  it  reaches  only  to  a  certain 
point,  upon  attaining  which,  the  growth  is  arrested,  and  never 
proceeds  farther.     The  arteries  cease  to  deposite  superabun- 

VoL.  I.  45 


354  THE  MECHANICAL  FUNCTIONS. 

dant  nourishment,  but  continue  to  maintain  an  exact  equili- 
brium between  the  expenditure  and  the  supply ;  so  that  the 
horns  of  the  cameleopard  are  never  shed,  and  remain  perma- 
nent bony  structures. 

A  farther  modification  of  this  process  occurs  in  the  con- 
struction of  the  horns  of  the  ox  and  of  the  sheep:  for  in 
these  the  bony  processes  arising  from  the  frontal  bones  are 
invested  with  a  covering  composed  of  horn,  the  nature  of 
which  is  totally  different  from  bone.  Two  tubercles  may 
be  seen  in  the  young  calf,  proceeding  from  the  bones  of  the 
forehead:  the  skin  covering  these  tubercles,  unlike  that 
which  precedes  the  antlers  of  the  deer,  is  unusually  thick 
and  hard.  As  the  skull  expands,  this  portion  of  integu- 
ment becomes  more  and  more  callous,  till  it  is  converted, 
by  the  action  of  the  subjacent  vessels,  into  a  solid,  hard, 
elastic,  and  insensible  fibrous  substance,  fitted  to  give  eflfect- 
ual  protection  to  the  subjacent  bony  layers  which  are  form- 
ing underneath  it.  The  highly  vascular  membrane,  from 
which  these  new  structures  chiefly  arise,  appears  to  have 
different  powers  of  production  at  its  two  surfaces:  for  while 
the  inner  surface  is  forming  the  osseous  portion  of  the  horn, 
and  supplying  the  phosphate  of  lime  required  for  the  con- 
struction of  its  plates  and  fibres,  the  exterior  surface  is  add- 
ing successive  layers  of  horny  substance  to  the  inner  side 
of  those  portions  which  had  been  before  deposited.  These 
two  operations,  w^hich  oflfer  a  remarkable  contrast,  both  as 
to  the  mode  of  their  performance,  and  as  to  the  nature  of 
the  resulting  products,  are  carried  on  at  the  same  time,  and 
by  the  same  organ,  but  on  different  sides.  The  bony  basis 
of  the  horn  is  an  organic  structure,  which  continues  to  be 
nourished  by  vessels  forming  part  of  the  general  system: 
the  horn  is  a  mere  excretion,  which  appears  to  be  destitute 
of  vessels,  and  is,  consequently,  removed  from  the  influence 
of  the  living  powers.  Thus  the  growth  of  horn  is  somewhat 
analagous  to  that  of  shell;  for  the  layers  which  compose  it 
are  deposited  in  succession;  each  new^  layer  is  agglutinated 
to  the  inner  surface  of  the  preceding;  and  each  has  the  shape 


RUMINANT  QUADRUPEDS. 


355 


of  a  hollow  cone,  occupying  the  part  towards  the  apex  of 
the  former  cone,  and  extending  farther  towards  the  base. 
Hence  a  longitudinal  section  of  the  whole  presents  the  ap- 
pearance represented  in  the  annexed  figures  (218*,)  where 
A  is  the  section  of  the  horn  of  an  Ox,  and  b,  a  similar  sec- 
tion of  the  horn  of  an  Antelope.  C  is  a  magnified  view  of 
the  extremity  of  the  latter,  together  with  a  portion  of  the 
bone  D,  which  occupies  the  axis  of  the  horn. 

In  this  process  of  the  formation  of  horn,  as  happens  in 
that  of  shells,  there  sometimes  occur  irregularities,  or  peri- 
odical intermissions  and  increase  of  action  in  the  secreting 
organs,  giving  rise  to  transverse  grooves,  or  ridges.  Tliese 
may  be  seen  in  the  horns  of  the  goat,  in  which  the  fibres 
are  short,  and  laid  one  over  another  with  the  same  regula- 
rity as  the  tiles  of  a  house.     The  tendency  in  these  horns 


to  assume  a  spiral  form  is  explicable  on  the  same  principles 
as  those  which  regulate  the  growth  of  turbinated  shells. 


356  THE  MECHANICAL  FUNCTIONS. 

The  horns  of  the  ox  and  of  the  antelope  tribes  are  formed 
of  longer  and  more  continuous  fibres,  which  are  closely 
compacted  together,  and  exhibit  very  distinctly  the  series 
of  hollow  cones  of  which  they  are  composed. 

The  horns  of  the  Rhinoceros,  both  of  the  one  and  two 
horned  species,  grow  from  the  integument  covering  the 
nose,  to  which  they  adhere  without  having  any  connexion 
with  the  subjacent  bones.  They  have  a  pyramidal  shape, 
and  are  composed  of  parallel  fibres,  resembling  hairs,  agglu- 
tinated together  into  a  solid  mass  by  a  material  which  acts 
as  a  cement.  This  fibrous  structure  is  most  distinctly  seen 
at  the  base  of  the  horn,  where  the  ends  of  the  fibres  project, 
like  those  of  a  brush,  from  the  surface.  When  these  horns 
are  sawn  transversely,  and  examined  with  a  magnifying 
glassy  a  great  number  of  orifices  are  seen,  marking  the 
empty  spaces  that  intervene  between  the  hairs;  and  if  the 
section  be  made  in  a  longitudinal  direction,  the  same  spaces 
give  rise  to  the  appearance  of  parallel  grooves.  These  horns 
are  not  deciduous,  like  those  of  the  stag:  but  continue  to 
adhere  to  the  skin,  and  to  grow  from  the  root,  in  proportion 
as  they  are  worn  at  the  extremity. 


§  6.  Solipeda, 

The  Solipeda  form  a  natural  family  of  quadrupeds,  in- 
cluding the  Horse,  the  *dss,  the  Quagga,  the  Zebra,  &c. 
which  are  very  nearly  allied  in  their  conformation  to  the 
ruminant  tribe.  To  combine  fleetness  w^ith  strength  has 
been  the  obvious  design  of  nature  in  the  construction  of 
these  animals.  We  find,  accordingly,  that  the  consolidation 
of  the  bones  of  the  foot  is  carried  still  farther  than  in  the 
ruminant  tribe;  for,  in  place  of  the  two  parallel  phalanges, 
which  are,  in  the  latter,  articulated  with  the  cannon  bone, 
there  is  here  only  a  single  metatarsal  bone.  The  three  pha- 
langes, of  which  that  single  finger  consists,  bear  the  names  of 
ihQ pastern,  the  coronet,  and  the  coffin  bo?ie;  and  the  hoof;  of 


MAMMALIA  SOLIPEDA.  ,  357 

course,  is  single,  likewise;  there  is,  also,  a  small  bone,  con- 
nected with  the  last,  and  called  the  shuttle  hone.  To  the 
cannon  bone  are  joined,  behind,  and  on  the  side,  two  much 
shorter  and  very  slender  bones,  which  are  rudiments  of  the 
other  metacarpal  bones.  They  have  been  termed  the  sty- 
loid, or  splint  hones;  and  are  generally  united  by  ossifica- 
tion with  the  cannon  bone.  The  scapula  of  the  horse  is 
very  narrow,  and  placed  very  nearly  in  a  straight  line  with 
the  humerus;  which  latter  bone  is  very  short,  and  scarcely 
descends  below  the  line  of  the  chest.  The  thigh-bone  is  also 
unusually  short.  The  muscles,  which  extend  the  joint,  and  'UJk 
throw  the  thigh  backwards,  in  kicking,  are  particularly  pow- 
erful. This  is  the  natural  defensive  action  of  the  horse:  and 
its  force  is  increased  by  a  particular  process  with  which  the 
bone  is  furnished,  and  which  has  the  form  of  a  strong  curved 
spine,  situated  on  the  outside,  and  opposite  to  the  lesser  tro- 
chanter,* giving  to  the  muscles  the  advantage  of  a  long  le- 
ver. The  cervical  vertebrae  have  only  short  spinous  pro- 
cesses, that  they  might  not  interfere  with  the  motions  of  the 
neck.  In  the  vertebrae  of  the  back,  on  the  other  hand,  these 
processes  are  remarkably  long,  especially  at  the  part  where 
the  shoulder  rests;  their  projection  constituting  what  is 
called  the  Withers. 


§  7.  Pachydermata, 

From  the  horse  we  pass,  by  a  natural  transition,  to  the 
Pachydermata^  a  small  group  of  animals  interesting  by  their 
peculiarities,  and  by  their  being  remnants  of  a  very  extensive 
tribe,  which  formerly  inhabited  the  earth,  but  have  now  al- 
most entirely  disappeared.  Although  they  feed  upon  grass, 
they  do  not  ruminate,  nor  are  they  cloven-footed.  They  are, 
for  the  most  part,  huge  and  unwieldy  animals,  with  thick  in- 
teguments, rendered  tough  by  a  large  mass  of  condensed  cel- 
lular substance,  which  forms  the  chief  defensive  armour  of 

*  This  process  has  been  termed  ^\q,  jiroccssus  recurvatusfemoris. 


358  THE  MECHANICAL  FUNCTIONS. 

those  that  are  destitute  of  either  tusk,  proboscis,  or  nasal 
horn. 

The  most  remarkable  genus  of  this  family  is  the  Elephant, 
the  colossal  giant  of  quadrupeds.  The  many  peculiarities  that 
are  observable  in  the  conformation  of  this  animal  have  all  an 
obvious  relation  to  the  circumstances  of  its  condition.  Formed 
for  feeding  on  a  great  variety  of  vegetable  substances,  and 
more  especially  on  the  tender  shoots  of  trees,  fruits,  and  grains, 
as  well  as  on  herbage,  and  succulent  roots,  its  organs  of  mas- 
tication are  powerful,  and  its  teeth  of  great  size.  The  whole 
of  this  apparatus  requires  an  immense  development  of  bone 
to  render  it  efficient ;  so  that  the  head,  with  its  huge  tusks 
and  grinders,  is  of  enormous  weight.  Had  this  ponderous  head 
been  suspended  at  the  end  of  a  neck  of  such  length  as  to  ad- 
mit of  its  being  carried  to  the  ground,  as  is  the  case  in  grazing 
animals,  it  would  have  destroyed  the  balance  of  the  body, 
and  would  have  required  greater  force  to  raise  and  retain  it 
in  a  horizontal  position  than  was  competent  to  any  degree  of 
muscular  power.  Nature  has  accordingly  abandoned  this 
form  of  structure,  and  has  at  once  curtailed  the  neck,  bring- 
ing the  head  close  to  the  trunk  of  the  body,  and  supporting  it 
by  means  of  short,  but  powerful  muscles,  which  are  not  im- 
planted in  any  particular  point  of  the  skull,  as  they  are  in 
other  quadrupeds,  where  the  occipital  bone  forms  a  crest  or 
ridge  for  that  purpose ;  but  the  general  surface  of  the  cranium 
has  been  enlarged  by  an  immense  expansion  given  to  its  inte- 
rior cellular  structure,  and,  thus,  the  muscles  are  attached  to 
a  considerable  extent  of  bone,  instead  of  being  affixed  to  a 
single  process,  which  would  have  incurred  great  risk  of  being 
broken  off  by  their  action.  These  large  cells  are  constructed 
with  a  view  to  combine  strength  with  lightness ;  the  plates 
which  form  their  sides  being  disposed,  in  a  radiated  manner, 
towards  the  circumference,  and  arranged  with  great  regulari- 
ty;  and  the  cells  themselves,  instead  of  containing  marrow,  are 
filled  with  air,  by  means  of  communications  with  the  Eusta- 
chian tubes,  which  open  into  the  nostrils :  thus,  a  great  extent 
of  surface  is  given  to  the  skull,  without  any  addition  to  its 


MAMMALIA  PACHYDERM  AT  A.  359 

weight.     The  hgamentum  niichae  also  comes  in  aid  of  the 
muscular  power,  being  here  of  vast  size  and  strength. 

The  head  being  limited  in  its  range  of  motion  by  its  approxi- 
mation to  the  trunk,  the  mouth  cannot  be  applied  directly  to 
seize  the  food :  and  some  means  were,  therefore,  to  be  pro- 
vided for  bringing  the  food  to  the  mouth.  For  this  purpose, 
a  new  organ,  the  proboscis,  has  been  constructed  :  it  consists 
of  a  cylinder,  perfectly  flexible,  and  of  a  length  sufficient  to 
reach  the  ground,  when  the  elephant  is  standing.  The  ani- 
mal has  the  power  of  moving  it  in  all  possible  directions,  by 
means  of  a  prodigious  number  of  muscular  fibres,  which  are 
collected  in  small  bands,  some  passing  transversely,  and  ra- 
diating from  the  interior  towards  the  circumference,  others  si- 
tuated more  obliquely,  and  a  third  set  running  longitudinally, 
and  forming  an  exterior  layer :  but  they  are  all  variously  in- 
terlaced together  so  as  to  compose  a  very  complicated  ar- 
rangement. The  extremity  of  the  proboscis,  w-hich  is  endowed 
with  great  sensibility,  is  furnished  with  an  appendix,  resem- 
bhng  a  finger,  most  of  the  functions  of  which,  indeed,  it  is 
capable  of  performing. 

For  the  formation  of  this  admirable  member  it  has  not 
been  necessary  to  deviate  from  the  ordinary  laws  of  deve- 
lopment by  the  creation  of  a  new  organ;  the  same  end  being 
accomplished  by  the  extension  of  a  structure  already  be- 
longing to  the  type  of  mammiferous  animals.  In  several  of 
the  pachydermata  the  nostrils  are  already  considerabl}^  ad- 
vanced, so  as  to  form  a  moveable  snout:  this  is  observable 
in  a  certain  degree  in  the  Hog;  it  is  still  more  remarkably 
seen  in  the  Tapir,  which  has  a  snout  so  lengthened  and  so 
moveable  as  very  much  to  resemble,  though  on  a  far  smaller 
scale,  the  proboscis  of  the  elephant.  This  latter  organ,  then, 
may  be  considered  as  merely  an  elongation  of  the  nostrils, 
which  have  been  drawn  out  to  suit  a  special  purpose,  very 
different  from  the  function  to  which  that  part  is  usually  sub- 
servient.^ 

♦  A  defective  development  of  the  bones  of  the  nasal  cavity,  while  the  na- 
tural growth  of  the  soft  parts  has  continued,  has  often,  in  the  case  of  thehu- 


360  THE  MECHANICAL  FUNCTIONS. 

While  fleetness  and  elasticity  are  the  results  of  the  me- 
chanical conformation  of  the  horse,  solidity  and  strength  are 
the  objects  chiefly  aimed  at  in  the  construction  of  the  pachy- 
dermata.  The  limbs  have  a  great  weight  to  sustain,  in  con- 
sequence of  the  huge  size  of  the  body;  and  hence  the  seve- 
ral bones  which  compose  the  pillars  for  its  support  are  ar- 
ranged nearly  in  vertical  lines.  The  joints  of  the  elbow  and 
knee  are  placed  low  from  the  body;  the  ulna  in  the  forelegs, 
and  the  fibula  in  the  hinder,  are  fully  developed,  and  are 
distinct  from  the  radius  and  the  tibia.  The  number  of  the 
toes,  instead  of  being  reduced  to  one,  as  in  the  horse,  or  to 
two,  as  in  ruminants,  is  here  increased  to  five:  though,  in 
consequence  of  their  being  very  short,  and  of  the  skin  which 
covers  and  surrounds  them  being  very  thick,  they  hardly 
appear  externally,  and  are  distinctly  recognised  only  in  the 
skeleton. 

It  would  carry  me  far  beyond  the  limits  of  the  present 
work,  were  I  to  engage  in  a  detailed  examination  of  all  the 
varieties  of  forms  and  structures  that  occur  in  the  mecha- 
nism of  the  different  tribes  of  mammalia,  in  reference  to  the 
purposes  they  are  intended  to  serve,  and  to  the  peculiar  cir- 
cumstances of  the  animal  to  which  they  belong.  I  must  ne- 
cessarily pass  over  a  multitude  of  instances  of  express  adap- 
tation, which  are  suited  only  to  particular  cases,  and  are, 
consequently,  of  minor  importance  as  regards  the  general 
plans  of  organization.  In  the  sort  of  bird's-eye  view  which 
I  am  taking  of  the  endless  modifications  of  structure  that 
have  been  executed  in  conformity  with  those  plans,  I  am 
able  particularly  to  notice  only  such  as  are  most  remarkable. 


§  8.  Rodentia, 

As  the  tribes  of  mammalia  we  have  hitherto  examined, 
employ  the  anterior  extremities  for  the  purposes  of  progres- 

man  foetus,  given  rise  to  a  monstrosity  veiy  much  resembling'  the  trunk  of  the 
tapir  or  of  the"  elephant.     (See  Geoffroy  St.  Hilaire.) 


MAMMALIA  RODENTIA.  361 

slon  only,  they  are  destitute  of  a  clavicle.  In  most  of  those 
which  follow,  and  where  a  greater  development  of  the  limb 
confers  more  extensive  and  more  varied  powers  of  motion, 
applicable  to  a  greater  range  of  objects,  this  bone  is  found. 
In  the  greater  number,  however,  it  is  merely  in  a  rudimental 
state ;  that  is,  developed  only  to  a  certain  extent,  one  portion 
being  bony,  and  the  rest  cartilaginous ;  as  if  the  ossification 
had  been  arrested  at  an  early  stage.  These  imperfect  clavi- 
cles are  too  short  to  connect  the  scapula  with  the  sternum ; 
the  rest  of  the  space  being  eked  out  by  cartilage,  and  by  li- 
gaments: but,  still,  they  are  of  great  use  in  affording  points 
of  attachment  to  the  muscles  of  the  limb,  and  giving  them  the 
advantage  of  acting  by  a  rigid  lever.  The  carnivorous  tribes, 
which  make  considerable  use  of  their  fore  paws  in  striking 
and  seizing  their  prey,  have  clavicles  of  this  description. 
Those  quadrupeds  which  have  to  execute  still  more  complex 
actions  with  their  fore  feet,  have  perfect  clavicles,  extending 
from  the  shoulder  to  the  chest,  and  connecting  the  bones  of 
the  anterior  extremity  with  the  general  frame-work  of  the 
skeleton.  This  is  the  case  in  a  large  proportion  of  the  family 
of  Rodentia,  such  as  the  Squirrel^  which  employs  its  paws 
for  holding  objects ;  and  the  Beaver,^  which,  likewise,  makes 
great  use  of  its  feet  for  moving  and  arranging  the  materials 
of  its  habitation.  Animals  that  dwell  in  trees,  and  require  to 
grasp,  with  force,  the  branches,  in  moving  along  them,  such 
as  the  Sloth,  have  also  distinct  clavicles.  Animals  which 
rake  or  dig  the  ground,  as  the  Mole,  the  Ant-eater,  and  the 
Hedge-hog,  are  all  provided  with  these  bones,  which,  by  keep- 
ing the  shoulders  at  the  same  constant  distance  from  the 
trunk,  and  affording  a  firm  axis  for  the  rotatory  motions  of 
the  limb,  materially  assist  them  in  the  performance  of  these 
actions. 

*  The  beaver  presents  a  singular  modification  in  the  structure  of  the  tail, 
which  is  expanded  into  a  flattened  oval  disk,  covered  by  a  skin  beset  with 
scales:  the  whole  forming-  a  mechanical  instrument,  which  maybe  compared 
to  a  trowel,  exceedingly  well  adapted  for  the  purposes  to  which  it  is  applied 
by  the  animal  in  constructing  its  mud  habitation. 

Vol.  I.  46 


362  THE  MECHANICAL  FUNCTIONS. 


§  9.  Lisectivora. 

In  the  tribe  of  Insectivorous  quadrupeds  we  meet  with 
several  races  which  present  singular  conformations.    In  none 
are  these  anomalies  more  remarkable  than  in  the  mole,  an 
animal  which  nature  has  formed  for  subterranean  residence, 
and  whose  limbs  are  constructed  with  a  view  to  the  rapid 
excavation  of  passages  under  ground.     The  hands  of  the 
mole,  for  its  fore  paws  almost  deserve  that  appellation,  are 
turned  upwards  and  backwards  for  scooping  the  soil,  while 
the  feet  are  employed  to  throw  it  out  with  great  quickness. 
These  mining  operations  are  aided  by  the  motions  of  the 
head,  which  is  lifted  with  great  power,  so  as  to  loosen  the 
ground  above,  and  overcomie  the  resistances  that  may  be  op- 
posed to  the  progress  of  the  animal.     That  no  impediment 
might  be  offered  to  these  motions  of  the  head,  the  spinous 
processes  of  the  cervical  vertebrae  have  not  been  suffered  to 
extend  upwards.   Large  muscles  are  provided  for  bending  the 
head  backwards  upon  the  neck;  and  they  are  assisted  by  a 
cervical  ligament  of  great  strength,  which  is  generally  in 
part  ossified.     The  muscles  of  the  fore  extremities  are  also 
of  extraordinary  power.     The  scapula  is  a  long  and  slender 
bone,  more  resembling  a  humerus  in  its  shape  than  an  ordi- 
nary scapula:  the  humerus,  on  the  contrary,  is  thick,  and 
square,  and  the  clavicle  is  short  and  broad.     The  radius  and 
the  ulna  are  distinct  from  each  other;  the  hand  is  very  large 
and  expanded;  the  palms  being  turned  outwards  and  back- 
wards, and  its  lower  margin  being  fashioned  into  a  sharp 
cutting  edge.     The  carpal  bones  and  the  phalanges  of  the 
fingers  are  very  much  compressed;  but  they  are  furnished 
with  large  nails,  which  compose  more  than  half  the  hands; 
and  they  are  expressly  constructed  for  digging,  being  long, 
broad,  and  sharp  at  the  extremities.     The  sternum  has  a 
large  middle  crest,  and  is  prolonged  at  its  extremity  into  a 
sharp  process,  having  the  figure  of  a  ploughshare,  thus  af- 


INSECTIVOROUS  MAMMALIA.  363 

fording  an  extensive  surface  of  attachment  for  the  large  pec- 
toral muscles,  from  which  the  limb  derives  its  principal 
force.  The  head  terminates  in  front  by  a  pointed  nOvSe, 
which  is  armed  at  its  extremity  with  a  small  bone,  intended 
to  assist  in  penetrating  through  the  ground. 

While  all  this  attention  has  been  paid  to  the  development 
of  the  anterior  part  of  the  body  to  which  these  instruments 
specially  contrived  for  burrowing  are  affixed,  the  hinder 
part  is  comparatively  feeble,  and  appears  stinted  in  its 
growth,  and  curtailed  of  its  fair  proportions.  The  pelvis  is 
exceedingly  diminutive,  being  reduced  to  a  slender  sacrum; 
and  it  is  thrown  far  back  from  the  abdomen,  to  which  it 
could  give  no  effectual  protection.  Hence  the  animal,  when 
above  ground,  w^alks  very  awkwardlj^,  and  is  unable  to  ad- 
vance but  by  an  irregular  and  vacillating  pace.* 

We  have  seen  that  there  is  a  tribe  of  fishes  armed  ex- 
ternally with  sharp  spines,  which  they  are  capable  of  erect- 
ing when  in  danger  of  attack.  A  similar  kind  of  defensive 
armour  is  furnished  to  the  Porcupine  and  the  Hedgehog, 
which  belong  to  the  family  of  insectivorous  quadrupeds. 
For  the  purpose  of  erecting  these  bristles,  when  the  animal 
is  irritated  or  alarmed,  there  is  provided  a  peculiar  set  of 
muscular  bands,  which  forms  part  of  the  usual  subcutaneous 
layer,  termed  the  pannicidus  carnosus.  In  the  hedgehog 
these  muscles  are  very  complicated,  and  give  the  animal 
the  power  of  rolling  itself  into  a  ball.  A  minute  descrip- 
tion of  these  muscles  has  been  given  by  Cuvier,  who  found 
that  the  whole  body  is  enveloped  in  a  large  muscular  bag, 
or  mantle,  lying  immediately  under  the  integuments;  and 
capable,  by  the  contraction  of  different  portions  of  its  fibres, 
of  carrying  the  skin  over  a  great  extent  of  surface.  In  the 
usual  state  of  the  animal,  this  broad  muscle  appears  on  the 

*  The  only  quadrupeds  which  resemble  the  mole  in  the  perfect  adaptation 
of  their  structure  to  the  pui-poses  of  burrowing",  are  the  Wombat  and  the 
Koala,  which  are  among"  the  many  extraordinary  animals  inhabiting-  the  con- 
tinent of  Australia.  Their  hind  leg's  are  constructed  in  a  manner  vei'y  mucli 
resembling-  the  human  fore-arm.     (See  Home,  Lectures,  See.  i.  134.) 


364  THE  MECHANICAL  FUNCTIONS. 

back  (as  represented  in  Fig.  219,)  contracted  into  a  thick 
oval  disk,  of  which  the  fibres  are  much  accumulated  at  the 
circumference.     From   the  edges   of  this  disk  there  pass 


down  auxiliary  muscles  towards  the  lower  parts  of  the  body; 
the  action  of  which  nyiscles  tends  to  draw  the  skin  down- 
wards, and  to  coil  it  over  the  head  and  paws,  in  the  manner 
shown  in  Fig.  220,  like  the  closing  of  the  mouth  of  a  great 
bag. 

§  10.  Carnivora, 

The  type  of  the  Mammalia  may  be  considered  as  having 
attained  its  full  development  in  the  carnivorous  tribes,  which 
comprehend  the  larger  beasts  of  prey.  As  their  food  is  ani- 
mal, they  acquire  a  less  complicated  apparatus  for  digestion 
than  herbivorous  quadrupeds,  possess  greater  activity  and 
strength,  and  enjoy  a  greater  range  of  sensitive  and  intel- 
lectual faculties.  In  accordance  with  these  conditions,  we 
may  notice  the  greater  expansion  of  their  brain,  the  su- 
perior acuteness  of  their  senses,  and  their  enormous  mus- 
cular power.  The  trunk  of  the  body  is  lighter  than  that 
of  vegetable  feeders,  especially  in  the  abdominal  region, 
and  is  compressed  laterally:  the  spine  is  more  pliant  and 
elastic,*  the  limbs  have  greater  freedom  of  motion,  the  ex- 
tremities  are  more  subdivided,  and  they  are  armed  with 

*  The  suppleness  of  the  spine  might  at  once  be  inferred,  on  the  simple  in- 
spection of  the  skeleton,  from  the  circumstance  that  the  vertebrae  of  the 
neck  and  loins  have  a  comparatively  small  development  of  their  spinous  pro- 
cesses. 


CARNIVOROUS  MAMMALIA.  365 

formidable  weapons  of  offence  and  destruction.  Great  me- 
chanical power  was  required  for  raising  the  head,  not  only 
on  account  of  the  force  to  be  exerted  in  tearing  flesh,  but 
also  that  these  animals  might  be  enabled  to  carry  away  their 
prey  in  their  mouths.  Hence  we  find  that  in  the  Lion,  of 
which  the  skeleton  is  represented  in  its  relations  to  the  out- 
line of  the  body,  in  Fig.  221,  the  first  vertebra  of  the  neck, 
or  atlas,  has  very  widely  expanded  transverse  processes, 
while  the  second  vertebra  has  a  largely  developed  spinous 
process,  for  supplying  levers  for  the  muscles  which  have  to 
perform  these  and  other  actions  in  which  the  head  is  con- 
cerned. 

The  whole  of  the  remaining  part  of  the  skeleton  of  these 
animals  is  constructed  with  reference  to  their  predatory  na- 
ture. The  sudden  springs  with  which  they  pounce  upon 
their  prey  must  impart  to  the  whole  osseous  frame  the  most 
violent  concussion.  The  first  stroke  with  which  they  attempt 
the  destruction  of  their  victims  is  given  with  the  fore  leg: 
so  that,  had  the  limb  been  rigidly  connected  with  the  ster- 
num, by  means  of  an  entire  clavicle,  its  motions  would  have 


been  too  limited,  and  danger  of  fracture  would  have  been  in- 
curred.    The  scapula  is  broad,  and  the   humerus  of  great 


366  THE  MECHANICAL  FUNCTIONS. 

length,  compared  with  the  same  bones  in  ruminants;  and  the 
latter  has,  besides,  a  large  surface  for  its  articulation  with 
the  former  of  these  bones,  thus  allowing  of  a  great  range  of 
motion:  the  radius  and  ulna  are  perfectly  distinct,  and  play 
extensively  on  each  other. 

The  fore  feet  rest  on  the  ground  by  means  of  the  second 
of  the  three  joints  of  which  each  toe  is  composed.  The  last 
phalanges  are  raised  at  right  angles  to  the  former,  for  the 
purpose  of  supporting  the  claws  in  an  erect  position.  It  has 
been  considered  of  such  importance  to  preserve  these  formi- 
dable instruments  constantly  sharp,  and  in  a  condition  fitted 
for  immediate  use,  that  an  express  contrivance  has  been  re- 
sorted to  for  this  purpose.  It  consists  in  a  sheath,  within 
which  the  claws,  when  not  employed,  are  kept  retracted,  by 
means  of  an  elastic  ligament,  which  constantly  tends  to  with- 
draw them  within  the  sheath:  and  they  are,  at  the  same  time, 
so  connected  with  the  tendons  of  the  flexor  muscles  of  the 
toes,  that  the  moment  these  muscles  are  throw^n  into  action, 
which  is  the  case  when  the  animal  aims  a  stroke  with  its 
paw,  the  claws  are  instantly  drawn  out,  and  combine  in  in- 
flicting the  severest  lacerations.* 

Connected  with  the  superior  strength  of  the  hind  extre- 
mities, we  find  the  pelvis  extending  farther  backwards,  and 
more  in  a  perpendicular  line  with  the  femur.  This  latter 
bone  is  longer  and  more  slender  than  in  the  horse,  but  it  is 
more  compact  in  its  form,  and  its  processes  are  more  strong- 
ly developed:  the  fibula  is  a  separate  bone  from  the  tibia. 
The  muscles,  in  general,  are  more  divided  into  portions,  and 
are  thus  capable  of  greater  diversity  of  action,  at  the  same 
time  that  they  have  greater  power  than  those  of  herbivorous 
quadrupeds.  The  articular  surfaces  are  of  greater  extent,  arid 
are  lubricated  with  a  more  copious  supply  of  synovia;  their 
ligaments  are  more  delicate  and  more  numerous;  and  the 

*  There  exists,  concealed  in  the  tuft  of  hair,  at  the  extremity  of  the  lion's 
tail,  a  small  conical  and  slightly  cui-ved  claw,  which  is  attached  to  the  skin 
only,  and  not  to  the  last  caudal  vertebra:  it  is  difficult  to  conjecture  what  can 
be  its  use. 


MAMMALIA  QUADRTJMANA.  367 

joints,  in  general,  adapted  to  a  greater  variety  of  movements. 
All  these  provisions  are  evidently  directed  to  confer  great 
freedom  and  facility  of  motion,  and  to  enlarge  the  sphere  of 
action  of  the  body  generally,  as  well  as  of  the  limbs. 


§  11.   Quadrumana. 

We  may  trace  in  the  series  of  quadrupeds  which  have 
come  under  our  review,  a  gradual  increase  in  the  develop- 
ment of  the  hind  feet:  beginning  from  the  horse,  which  is 
single  hoofed,  or  solipede;  next  to  which  rank,  the  cloven- 
footed  ruminants,  a  tribe  which  includes  the  camel,  whose 
foot  is  widely  expanded,  for  the  purpose  of  treading  secure- 
ly on  sand;  then  come  the  Rhinoceros,  which  has  three 
hoofed  toes;  the  Hippopotamus,  which  has  four;  and  the 
Elephant,  which  has  five.  To  these  succeed  another  series, 
where  nails,  or  claws,  are  substituted  for  hoofs,  as  is  the  case 
with  all  the  Carnivora,  which,  standing  on  the  extremities 
of  their  toes,  have  been  termed  Digitigrades.  Then  follow 
the  Plantigrade  quadrupeds,  such  as  the  bear,  the  badger, 
the  hedgehog  and  the  mole,  which  rest  with  the  whole  foot 
on  the  ground,  and  are,  in  consequence,  able  to  make  great 
use  of  their  fore  paws.  These  conduct  us  to  the  family  of 
the  Quadrumana^  comprehending  the  Monkey  and  the  Le- 
mur tribes,  which  are  characterized  by  having  the  inner  toe 
quite  distinct  from  the  others,  like  the  human  thumb,  and 
which  appear,  therefore,  as  if  they  had  four  hands. 

The  Quadrumana  present  the  nearest  approximation  to  the 
human  structure:  they  are  naturally  inhabitants  of  the  forest, 
and  their  conformation  is  adapted  to  the  actions  of  climbing 
upon  trees,  of  grasping  the  branches,  and  of  springing  from 
the  one  to  the  other,  with  precision  and  agility.  It  is  here 
that  they  are  at  home;  it  is  here  that  they  gather  the  food 
which  is  most  suited  to  their  nature;  it  is  here  that  they  en- 
gage in  successful  combats  with  serpents  and  other  enemies; 
retaining  their  positions  in  perfect  security  on  the  moving 


368  THE   MECHANICAL  FUNCTIONS. 

branches,  or  sportively  swinging  by  their  extremities  in  the 
air.  Both  the  feet  and  the  hands  are  formed  for  this  species 
of  prehension;  and  many  are  farther  provided  with  a  strong- 
ly prehensile  tail,  which  is  an  instrument  admirably  adapted 
for  all  these  purposes.  Hence,  the  attitude  most  natural  to 
these  animals  is  neither  the  horizontal  one  of  quadrupeds, 
nor  the  erect  posture  of  man,  but  an  intermediate  or  semi- 
erect  position. 

This  view  of  the  living  habits  of  the  quadrumana  will  af- 
ford the  key  to  most  of  the  peculiarities  of  structure  they 
present  to  our  observation.  The  head,  being  no  longer  sus- 
pended at  the  end  of  a  horizontal,  or  recurved  neck,  is,  in 
the  usual  attitude  of  the  animalj  supported  chiefly  by  the 
cervical  vertebrae.  The  greater  development  of  the  brain, 
and,  miore  especially,  of  its  posterior  lobes,  creates  a  neces- 
sity for  an  extension  of  the  occipital  bone  in  that  direction, 
a  portion  of  the  weight  to  be  sustained  by  the  atlas,  is,  ac- 
cordingly, thrown  behind  the  centre  of  motion,  which  is  at 
its  articulation  with  the  latter  bone;  and  this  weight  tends, 
therefore,  to  balance  that  of  the  anterior  part  of  the  head; 
hence,  there  is  no  need  of  the  strong  cervical  ligament,  which 
is  so  universally  met  with  in  quadrupeds,  and,  although  this 
ligament  exists  in  the  monkey,  it  is  very  slender,  and  of  no 
very  great  extent. 

Great  mobility  has  been  conferred  on  the  spine  by  the 
form  of  its  articulations;  and  the  caudal  vertebrae  are  gene- 
rally greatly  multiplied  to  form  a  tail  of  considerable  length, 
which,  in  the  Jiteles,  or  spider  monkey  of  America,  is 
moved  by  powerful  muscles,  and  is  an  organ  of  great  flexi- 
bility and  strength.  Monkeys  possess  a  distinct  clavicle,  a 
lengthened  humerus  and  a  femur,  a  radius  and  ulna  movea- 
ble upon  each  other,  and  a  hand  nearly  approaching  to  the 
human  construction.  But  the  thumb  is  less  developed,  and 
its  muscles  are  much  weaker  than  in  man. 

The  bones  of  the  pelvis,  as  well  as  those  of  the  leg,  are 
elongated,  for  the  purpose  of  giving  greater  length  to  the  mus- 
cles which  are  to  move  their  several  parts;  by  this  means, 


THE  HUMAN  FRA3IE.  3G9 

although  the  force  with  which  they  act  may  be  somewhat 
lessened,  yet  the  velocity  of  the  motion  they  produce  is  in- 
creased in  the  same  proportion.     The  fibula  is  here  a  bone 
of  more  importance  than  in  quadrupeds;  for  it  performs  a 
motion  of  rotation  round  the  tibia,  analogous  to  that  of  the 
radius  upon  the  ulna,  giving  a  great  extent  of  action  to  the 
foot,  and  converting  the  leg  into  an  arm,  as  we  have  already 
seen  that  the  foot  itself  is  transformed  into  a  hand.    A  small 
inclination  is  given  to  the  articulation  of  the  tarsus  with  these 
last  mentioned  bones,  which  imparts  a  degree  of  twist  to  the 
feet,  throwing  the  sole  inw^ards,  and  causing  the  monkey 
while  walking  to  rest  chieflj^on  its  outer  edge.    This  seem- 
ing defect  gives  a  slight  appearance  of  awkwardness  to  the 
gait;  it  is  not,  however,  to  be  view^ed  as  an  imperfection^  for 
it  is  evidently  designed  to  assist  the  animal  in  climbing  trees, 
which  is  its  most  usual  action,  the  oblique  position  of  the 
foot  enabling  it  most  effectually  to  lay  hold  of  the  branches. 
Monkeys  are  evidently  not  formed  to  excel  in  swiftness;  for 
the  heel,  in  these  animals,  presents  no  large  projection  as  in 
other  orders  of  mammalia;  nor  are  the  muscles  that  are  in- 
serted into  the  heel  particularly  powerful;  they  hardly,  in^ 
deed,  can  be  said  to  compose  a  calf  as  in  the  human  leg. 

§  12.  Man. 

The  series  of  structures  modelled  on  the  characteristic 
type  of  the  Mammalia,  after  having  exhibited  the  successive 
development  of  all  its  elements,  attains  the  highest  perfec- 
tion in  the  human  fabric;  for  even  independently  of  those 
prerogatives  of  intellect  and  of  sensibility,  by  which  Man 
is  so  far  exalted  above  the  level  of  the  brute  creation,  both 
his  ph3^sical  structure  and  his  physiological  constitution  place 
him  incontestably  at  the  summit  of  tlie  scale  of  terrestrial 
beings.  Considered  zoologically,  indeed,  the  human  species 
must  rank  among  the  Mammalia,  and  it  even  makes  a  near  ap- 
proach to  the  Quadrumana;  yet  there  exists  many  peculiari- 
ties of  structure,  which  entitle  Man  to  be  placed  in  a  sepa- 

Vol.  I.  47 


370  THE  MECHANICAL  FUNCTIONS. 

rate  order,  where  disclaiming  any  close  alliance  with  inferior 
creatures,  he  proudly  stands  alone,  towering  far  above  them 
all. 

It  is  not,  however,  on  a  pre-eminence  in  any  single  phy- 
sical quality  or  function  that  this  title  to  superiority  can  be 
founded;  for  in  each  of  these  endowments  man  is  excelled 
in  turn  by  particular  races  of  the  lower  animals;  but  the 
chief  perfection  of  his  frame  consists  in  its  general  adapta- 
tion to  an  incomparably  greater  variety  of  objects,  and  an  infi- 
nitely more  expanded  sphere  of  action.  As  the  beauty  of  an 
edifice  depends  not  on  the  elaborate  finishing  of  any  one  por- 
tion, but  results  from  the  general  suitableness  of  the  whole 
to  the  purposes  for  which  it  was  constructed,  so  the  excellence 
of  the  human  fabric  is  to  be  estimated  by  the  exquisite  pro- 
portion and  harmony  subsisting  among  all  its  parts,  and  per- 
vading the  whole  system  of  its  functions.  The  design  of 
its  structure  and  economy  embraces  widely  different,  and  far 
higher  aims  than  those  contemplated  in  the  organization  of 
any  of  the  inferior  animals.  Destined  to  an  intellectual,  a 
social,  and  a  moral  existence,  Man  has  had  every  part  of  his 
organization  modified  with  an  express  relation  to  these  great 
objects  of  his  formation.  This  will  best  appear  when  we 
come  to  examine  the  organs  which  are  subservient  to  the 
sensitive  and  active  faculties;  but  even  here,  where  our  views 
must,  for  the  present,  be  limited  to  the  mechanical  circum- 
stances of  his  structure,  the  proofs  are  sufficiently  numerous 
to  warrant  this  general  conclusion. 

Man  presents  the  only  instance  among  the  mammalia  of 
a  conformation  by  which  the  erect  posture  can  be  perma- 
nently maintained,  and  in  which  the  office  of  supporting  the 
trunk  of  the  body  is  consigned  exclusively  to  the  lower  ex- 
tremities. To  this  intention  the  form  and  arrangement  of 
all  the  parts  of  the  osseous  fabric,  and  the  position  and  ad- 
justments of  the  organs  of  sense  have  a  well  marked  refer- 
ence.*    The  lower  limbs  are  qualified  to  be  the  efficient 

*  In  most  quadrupeds,  as  we  have  seen,  the  thorax  is  deep  in  the  direc- 
tion fi-om  the  sternum  to  the  spine,  but  is  compressed  latemlly,  for  the  evi- 


THE  HUMAN  FRAME.  371 

instruments  of  progression  by  their  greater  length  and  mus- 
cularity, compared  with  the  generality  of  quadrupeds.  The 
only  exceptions  to  this  rule  occur  in  those  mammalia  which 
are  constructed  expressly  for  leaping,  such  as  the  Kanguroo 
and  Jerboa^  where,  however,  the  hind  legs  are  employed 
almost  solely  for  that  mode  of  progression.  The  Quadru- 
mana,  which  come  nearer  to  the  human  form  than  any  of 
the  other  tribes,  have  the  lower  limbs  comparatively  weak. 
In  almost  all  other  quadrupeds  the  disproportion  is  still 
greater,  the  thigh  being  short,  and  almost  concealed  by  the 
muscles  of  the  trunk,  and  the  remainder  of  the  limb  being 
slender,  and  not  surrounded  by  any  considerable  mass  of 
muscles. 

The  articular  surfaces  of  the  knee  joint  are  broader,  and 
admit  of  greater  extent  of  motion  in  man  than  in  quadru- 
peds: hence  the  leg  can  be  brought  into  the  same  line  with 
the  thigh,  and  form  with  it  a  straight  and  firm  column  of 
support  to  the  trunk;  and  the  long  neck  of  the  thigh  bone 
allows  of  more  complete  rotation.  The  widely  spread  basin 
of  the  pelvis  effectually  sustains  the  weight  of  the  digestive 
organs,  and  they  rest  more  particularly  upon  the  broad  ex- 
pansion of  the  iliac  bones:  in  quadrupeds,  these  bones,  havino- 
no  such  weight  to  support,  are  much  narrower. 

The  base  on  which  the  whole  body  is  supported  in  the 
erect  position  is  constituted  by  the  toes,  and  by  the  heel, 
the  bone  of  which  projects  backwards  at  right  angles  to  the 
leg.  Between  these  points  the  sole  of  the  foot  has  a  conca- 
vity in  two  directions,  the  one  longitudinal,  the  other  trans- 
verse, constituting  a  double  arch.  This  construction,  be- 
sides "conferring  strength  and  elasticity,  provides  room  for 
the  convenient  passage  of  the  tendons  of  the  toes,  which 
proceed  downwards  from  the  larger  muscles  of  the  leg,  and 

dent  purpose  of  brlng-Ing-  the  fore  limbs  nearer  to  each  other,  that  they 
might  more  effectually  support  the  anterior  part  of  the  trunk.  In  Man,  on 
the  contrary,  the  thorax  is  flattened  anteriorly,  and  extends  more  in  widtli 
than  in  depth}  thus  throwing-  out  the  shoulders,  and  allowing  an  extensive 
range  of  motion  to  the  arms. 


■>ts 


372  THE  MECHANICAL  FtrNCTIONS. 

also  for  the  lodgement  of  smaller  muscles  affixed  to  each  in- 
dividual joint,  and  for  the  protection  of  the  various  nerves 
and  blood  vessels  distributed  to  all  these  parts.  The  con- 
cavity of  the  foot  adapts  it  also  to  retain  a  firmer  hold  of  the 
inequalities  of  the  ground  on  which  we  tread.  The  muscles 
which  raise  the  heel,  and  which  compose  the  calf  of  the 
leu,  are  of  creat  size  and  streno;th,  and  derive  a  considerable 
increase  of  power  from  the  projection  of  the  bone  of  the 
heel,  into  which  their  united  tendons  are  inserted.  In  all 
these  respects  the  human  structure  possesses  decided  advan- 
tages over  that  of  the  monkey,  with  reference  to  the  specific 
objects  of  its  formation. 

It  is  impossible  to  doubt  that  nature  intended  man  to  as- 
sume the  erect  attitude,  when  we  advert  to  the  mode  in 
which  the  head  is  placed  on  the  spinal  column.  The  enor- 
mous development  of  the  brain,  and  of  the  bones  which  in- 
vest it,  increases  so  considerably  the  weight  of  that  part  of 
the  head,  which  is  situated  behind  its  articulation  with  the 
vertebrse  of  the  neck,  that  the  balance  of  the  whole  is  much 
more  equal  than  it  is  in  the  monkey,  where  the  weight  of 
the  fore  part  very  greatly  preponderates.  The  muscles 
which  bend  the  head  back  upon  the  neck,  and  retain  it  in 
its  natural  position,  are  therefore  not  required  to  be  so  pow- 
erful as  they  must  be  in  quadrupeds,  especially  in  those 
which  graze,  and  in  which  the  mouth  and  eyes  must  fre- 
quently be  directed  downwards,  for  the  purpose  of  procuring 
food.  In  man  this  attitude  would,  if  continued,  be  extreme- 
ly fatiguing,  from  the  weakness  of  those  muscles,  and  the 
absence  of  that  strong  ligament  which  sustains  the  w^elght 
of  the  head  in  the  ordinary  horizontal  attitude  of  quadru- 
peds. 

"Pronaque  cum  spcclant  anlmalia  cxtera  terram, 
Os  hominl  sublime  dedit,  cxlumque  tueri 
Jusslt,  et  erectos  ad  sidera  toilere  vultus." — Otid. 

The  space  comprehended  by  the  two  feet  is  extremely 
narrow,  when  compared  with  the  extended  base  on  which 
the  quadruped  is  supported.  Hence,  the  stability  of  the  body 


THE  HUMAN  FRAME.  S*!? 

must  be  considerably  less.     The  statue  of  an  elephant,  placed 
upon  a  level  surface,  would  stand  without  danger  of  overset- 
ting: but  the  statue  of  a  man,  resting  on  the  feet,  in  the  usual 
attitude  of  standing,  Avould  he  thrown  down  by  a  very  small 
impulse.     It  is  evident,  indeed,  that  in  the  living  body,  if 
the  centre  of  gravity  were  at  any  moment  to  pass  beyond 
the  base,  no  muscular  effort  which  could  then  be  made,  would 
avail,  to  prevent  the  body  from  falling.     But  the  actions  of 
the  muscles  are  continually  exerted  to  prevent  the  yielding 
of  the  joints  under  the  weiglit  of  the  body,  which  tends  to 
bend  them.     In  quadrupeds,  less  exertion  is  requisite  for 
that  purpose;  and  standing  is  in  them,  as  we  have  seen,  a 
posture  of  comparative  repose:  in  man  it  requires  nearly  as 
great  an  expenditure  of  muscular  power  as  the  act  of  walk- 
ing.    Soldiers,  on  parade,  experience  more  fatigue  by  re- 
maining in  the  attitude  of  standing,  than  they  would  by 
marching,  during  an  equal  time.     Strictly  speaking,  indeed, 
it  is  impossible  for  even  the  strongest  man  to  remain  on  his 
legs,  in  precisely  the  same  position,  for  any  considerable 
length  of  time.    The  muscles  in  action  soon  become  fatigued, 
and  require  to  be  relieved  by  varying  the  points  of  support, 
so  as  to  bring  other  muscles  into  play.     Hence,  the  weight 
of  the  body  is  transferred  altcrnatel}'  from  one  foot  to  the 
other.     The  action  of  standing  consists,  in  fact,  of  a  series  of 
small  and  imperceptible  motions,  by  which  the  centre  of 
gravity  is  perpetually  shifted  from  one  part  of  the  base  to 
another;  the  tendency  to  fall  to  any  one  side  being  quickly 
counteracted  by  an  insensible  movement  in  a  contrary  direc- 
tion.    Long  habit  has  rendered  us  unconscious  of  these  ex- 
ertions, which  we  are,  nevertheless,   continually  making; 
but  a  child  learning  to  walk  finds  it  difficult  to  accomplish 
them  successfully.     It  is  one  among  those  arts  which  he  has 
to  acquire,  and  which  costs  him,  in  the  apprenticeship,  many 
painful  efforts,  and  many  discouraging  falls.     But  whenever 
nature  is  the  teacher,  the  scholar  makes  rapid  progress  in 
learning;  and  no  sooner  have  the  muscles  acquired  tJie  ne- 
cessary strength,  than  the  child  becomes  an  adept  in  ba- 


374  THE  MECHANICAL  FUNCTIONS. 

lancing  its  body  in  various  attitudes,  and,  in  a  very  short 
time,  is  unconscious  that  these  actions  require  exertion. 

In  walkins:,  the  first  effort  that  is  made  consists  in  trans- 
ferring  the  whole  weight  of  the  body  upon  one  foot,  with  a 
view  to  fix  it  on  the  ground;  and,  then,  the  other  foot,  being 
at  liberty,  is  brought  forwards.  By  this  action,  the  centre 
of  gravity  is  made  to  advance,  till  it  passes  beyond  the  base 
of  the  foot:  in  this  situation,  the  body,  being  unsupported, 
falls  through  a  certain  space,  and  would  continue  its  descent, 
were  it  not  that  it  is  received  on  the  other  foot,  wjiich,  by 
this  time,  has  been  set  upon  the  ground.  This  falling  of  the 
body  would,  if  not  immediately  checked,  become  very  sen- 
sible; as  happens  when,  on  walking  inattentively,  the  foot 
we  had  advanced  comes  down  to  a  lower  level  than  we  were 
prepared  for;  in  which  case,  the  body,  having  acquired  a 
certain  velocity  by  its  greater  descent,  receives  a  sudden 
shock  when  that  velocity  is  checked,  and  thus  a  disagreea- 
ble jar  is  given  to  the  whole  frame. 

While  the  weight  of  the  body  is  thus  transferred,  alter- 
nately, from  one  foot  to  the  other,  the  centre  of  gravity  not 
only  rises  and  falls,  so  as  to  describe,  at  every  step,  a  small 
arch,  but  also  vibrates  from  side  to  side,  so  that  the  series  of 
curves  it  describes,  are  somewhat  complicated  in  their  form. 
This  undulation  of  the  body,  from  one  foot  to  the  other, 
would  scarcely  ever  be  performed  w^ith  perfect  equality  on 
both  sides,  if  we  trusted  wholly  to  the  sensations  communi- 
cated by  the  muscles,  and  if  we  were  not  guided  by  the  sense 
of  sight,  or  some  other  substitute.  Thus,  a  person  blind- 
folded cannot  walk  far  in  a  straight  line;  for,  even  on  a  le- 
vel plane,  he  will  incline  unconsciously  either  to  the  right 
or  to  the  left. 

In  all  quadrupeds,  and  even  also  in  the  quadrumana,  the 
fore  extremities  more  or  less  contribute  to  the  support  and 
progression  of  the  body:  it  is  only  in  man  that  they  are 
wholly  exempted  from  these  ofhces,  and  are  at  liberty  to  be 
applied  to  other  purposes,  and  employed  as  instruments  of 
prehension  and  of  touch.     In  the  power  of  executing  an  in- 


THE  HUMAN  FRAME.  375 

finite  variety  of  movements  and  of  actions,  requiring  either 
strength,  delicacy,  or  precision,  the  human  arm  and  hand, 
considered  in  their  mechanism  alone,  are  structures  of  unri- 
valled excellence;  and,  when  viewed  in  relation  to  the  in- 
tellectual energies  to  which  they  are  subservient,  plainly  re- 
veal to  us  the  divine  source,  from  which  have  emanated  this 
exquisite  workmanship,  and  these  admirable  adjustments,  so 
fitted  to  excite  in  our  breasts  tlie  deepest  veneration,  and  to 
fill  us  with  never  ceasing  wonder. 

To  specify  all  the  details  of  express  contrivance  in  the 
mechanical  conformation  of  the  hand  would  alone  fill  a  se- 
parate treatise:  but  I  must  refrain  from  pursuing  this  inte- 
resting subject,  as,  fortunately,  the  task  ]|as  devolved  upon 
one  far  more  able  than  myself  to  do  it  justice. 


(     376     ) 


CHAPTER  X. 

VERTEBRATA  CAPABLE  OP  FLYING. 

§  1.   Verlebrata  ivilhout  Feathers,  for^med  for  flying. 

Few  problems  in  mechanic  art  present  greater  practical 
difficulties  than  thft  of  raising  from  the  ground,  and  of  sus- 
taining and  moving  rapidly  through  the  air  an  animal  body^ 
composed  as  it  must  be  of  many  ponderous  organs,  that  are 
requisite  for  the  performance  of  the  higher  functions  of  life: 
yet  Nature  has  achieved  all  this,  not  only  in  endless  tribes 
of  the  more  diminutive  invertebrate  animals,  but  also  in  the 
more  solid  and  massive  organizations  which  are  modelled  on 
the  vertebrate  type.  These  objects  have  been  accomplished, 
in  all  cases,  without  the  employment  of  any  other  than  the 
ordinary  elements  of  those  organizations;  modified,  indeed, 
to  suit  the  particular  purpose  in  view,  but  yet  essentially  the 
same,  and  regulated  by  the  same  laws  of  development  which 
prevail  throughout  the  whole  animal  system.  The  adapta- 
tion of  these  elements  to  the  construction  of  an  apparatus  of 
so  refined  a  nature  as  that  which  is  required  for  flying,  im- 
plies the  deepest  foresight,  the  most  extensive  plan,  and  the 
most  artificial  combination  of  means.  The  foundations  for 
these  peculiar  forms  of  mechanism  are  laid  in  the  primeval 
constitution  of  the  embryo;  and  a  long  and  curious  series  of 
preparatory  changes  must  take  place  before  the  completion 
of  the  finished  structures.  Of  this  we  have  already  had  a 
remarkable  example  in  the  metamorphoses  of  insects,  which 
exhibit,  in  their  last  stage  of  development,  the  highest  de- 
gree of  perfection  compatible  with  the  articulate  type. 
Birds,  in  like  manner,  present  us  with  the  highest  refine- 


POWER  OF  FLYING.  377 

ment  of  mechanical  conformation  which  can  be  attained  by 
the  development  of  a  vertcbratcd  structure. 

The  power  of  flying  is  derived  altogether  from  the  resist- 
ance which  the  air  opposes  to  bodies  moving  throuo-h  it   or 
acting  upon   it  by   mechanical   impulse.     In   the  ordinary 
movements  of  our  own   bodies,  this  resistance  is  scarcely 
sensible,  and  hardly  ever  attracts  notice:  but  it  increases  in 
proportion  to  the  surface  which  acts  upon  the  air,  and  still 
more  according  to  the  velocity  of  the  moving  body;  for  the 
increase  is  not  merely  in  the  simple  ratio  of  the  velocity, 
but  as  its  square,  or  perhaps,  even  a  higher  power.    In  order 
that  an  animal  may  be  able  to  fly,  therefore,  two  principal 
conditions  are  required:  there  must,  first,  be  a  considerable 
extent  of  surface  in  the  wings,  or  instruments  which  act 
upon  the  air;  and  there  must,  secondly,  be  sufficient  mus- 
cular power  to  give  these  instruments  a  very  great  velocity. 
Both  these  advantages  are  found  combined  in  the  anterior 
extremities  of  birds,  and  no  animals  belonging  to  any  other 
class  possess  them  in  the  same  perfection.     No  quadruped, 
except  the  bat,  has  sufficient  muscular  power  in  its  .limbs, 
however  aided  by  an  expansion  of  surface,  to  strike  the  air 
with  the  force  requisite  for  flight.     No  refinement  of  me- 
chanic ingenuity  has  ever  placed  the  Daedalian  art  of  flying 
within  the  reach  of  human  power;  for  even  if  the  lightest 
possible  wings  could  be  so  artificially  adapted  to  the  body  as 
to  receive  the  full  force  of  the  actions  of  the  limbs,  however 
these  actions  might  be  combined,  they  would  fall  very  far 
short  of  the  exertion  necessary  for  raising  the   body  from 
the  ground. 

Examples,  however,  occur  in  every  one  of  the  classes  of 
vertebrated  animals,  where  an  approach  is  made  to  this  fa- 
culty. In  the  Exocastus,  or  flying-fish,  the  pectoral  fins  have 
been  enormously  expanded,  evidently  for  the  purpose  of  en- 
abling the  animal  to  leap  out  of  the  water,  and  support  itself 
for  a  short  interval  in  the  air:  but  its  utmost  efforts  are  inade- 
quate to  sustain  it  beyond  a  few  moments  in  that  element, 
Vol.  I.  48 


37S  THE  MECHANICAL  FUNCTIONS. 

and  it  can  never  rise  to  more  than  five  or  six  feet  above  the 
surface  of  the  water. 

A  species  of  lizard,  called  the  Draco  Fb/a7Z5,  has  a  singu- 
larly constructed  apparatus,  which  appears  like  two  wings, 
affixed  to  the  sides  of  the  back,  and  quite  independent  of 
either  the  fore  or  the  hind  extremities.     By  the  aid  of  these 
moveable  flaps,  the  animal  is  able  to  descend  from  the  tops 
of  trees,  or  flutter  lightly  from  branch  to  branch;  but  this  is 
the  utmost  that  it  can  accomplish  by  means  of  these  imper- 
fect organs.     The    construction  of  these  anomalous  mem- 
bers is  highly  curious  in  a  physiological  point  of  view;  as 
showing  how  Nature,  in  effecting  a  new  purpose,  is  in- 
clined   to  resort  to  the  modification  of  structures  already 
established  as  constituent  parts  of  the  frame,  in  preference 
to  creating  new  organs,  or  such  as  have  no  prototype  in  the 
model  of  its  formation.    Frequent  proofs  of  this  law,  indeed, 
are  afforded  by  the  comparative  examination  of  the  anatomy 
of  the  organs  of  progressive  motion.    The  ribs,  in  particular, 
are  often  the  subject  of  these   conversions  to  uses  very  dif- 
ferent from  their  ordinary  function,  which  is  that  of  assist- 
ing in  respiration.    Thus,  we  have  seen  that  in  the  Tortoise 
they  are  expanded  to  form  the  carapace,  uniting  with  corre- 
sponding dilatations  of  the  sternum,  and  sterno-costal  appen- 
dages, in  composing  a  general  osseous  ineasement  to  the 
body.     In  Serpents,  again,  the  ribs  are  employed  as  organs 
of  progressive  motion;  performing  the  functions  of  legs,  and 
having  affixed  to  their  extremities  the  abdominal  scuta,  by 
way  of  feet.     The  cervical  ribs  of  the  Cobra  de  Capello,  or 
hooded  snake  of  the  East  Indies,  are  employed  for  the  me- 
chanical purpose  of  supporting  an  expansion  of  the  skin  of 
the  neck,  which  forms  a  kind  of  hood,  capable    of  being 
raised  or  depressed  at  the  pleasure  of  the  animal."^     These 
ribs  are  entirely  unconnected  with  the  respiration  of  the  ser- 
pent. 

In  the  Draco  volans,  which  was  to  be  furnished  with  rii- 

*  Phil.  Trans,  for  1804,  p.  346. 


FLYING  LIZARD. 


379 


struments  for  assisting  it  in  its  distant  leaps  through  the  air, 
it  is  again  the  ribs  which  are  resorted  to  for  furnisliing  the 
basis  of  such  an  apparatus.  On  each  side  of  the  dorsal  ver- 
tebrae, as  is  seen  in  the  skeleton  of  this  animal  (Fig.  222,) 
the  eight  posterior  ribs  on  each  side,  instead  of  having  the 
usual  curvature  inwards,  and  instead  of  being  continued 
round  to  encircle  the  body,  are  extended  outwards  and  elon- 
gated, and  are  covered  with  a  thin  cuticle,  derived  from  the 
common  integuments.  The  ordinary  muscles  which  move 
the  ribs  still  remain,  but  with  greatly  increased  power,  and 
serve  to  flap  these  strangely  formed  wings  at  the  pleasure  of 
the  animal,  during  its  short  aerial  excursions. 


222 


Among  the  mammalia,  we  meet  with  a  few  species  which 
have  a  broad  membrane,  formed  of  a  duplicature  of  the  skin, 
extended  like  a  cloak  from  the  fore  to  the  hind  extremities, 
and  enabling  the  animal  to  flutter  in  the  air,  and  to  break  its 
fall  during  its  descent  from  the  branches  of  trees.  Struc- 
tures of  this  kind  are  possessed  by  the  Sciurus  volans^  or 


380 


THE  MECHANICAL  FUNCTIONS. 


flying  squirrel,  and  also  by  some  other  species  of  the  same 
genus.  They  are  seen  on  a  still  larger  scale  in  the  Lemur 
volans,  or  Galeopitheciis.  The  resistance  which  these  broad 
expansions  of  skin  oppose  to  the  air,  when  the  limbs  are 
spread  out,  enables  the  animal  to  descend  in  perfect  safety 
through  that  medium  from  very  considerable  heights:  but 
these  appendages  to  the  body  are  mere  parachutes,  not  wings, 
and  none  of  the  animals  which  possess  them  can,  by  their 
means,  and  with  the  utmost  efforts  which  their  muscles  are 
capable  of  exerting,  ever  rise  from  the  ground,  or  even  sus- 
pend themselves  for  a  moment  in  the  air. 

The  only  quadruped  that  can  properly  be  said  to  be  en- 
dowed with  the  power  of  flying  is  the  Bat.  In  this  animal 
the  portions  of  the  skeleton  (f.  Fig.  223)  which  correspond 


to  the  phalanges  of  the  fingers  are  extended  to  an  enormous 
length,  and  the  pectoral  muscles,  which  move  the  anterior 
extremities,  are  of  extraordinary  size  and  power.  In  the 
larger  species,  each  wing  is  at  least  two  feet  in  length.  The 
fine  membrane,  which  is  spread  between  these  lengthened 
fingers,  has  its  origin  in  the  sides  of  the  neck,  and  reaches 
all  along  the  body  to  the  extremities  of  the  hinder  legs, 
which  it  encloses  in  its  folds.  Thus,  not  only  is  the  sur- 
face, by  which  it  acts  upon  the  air,  sufiiciently  extensive, 
but  the  muscular  powder,  by  which  its  motions  are  effected, 
is  adequate  to  give  it  those  quick  and  sudden  impulses  which 
are  requisite  for  flying:  and  thus,  although  its  structure  is 
totally  different  from  that  of  birds,  it  yet  performs  fully  the 


BAT.  381 

office  of  a  real  wing.  The  bat  flies  with  perfect  ease,  even 
while  carrying  along  with  it  one  or  two  of  its  young:  it  is 
not,  however,  fitted  for  very  long  flights. 

The  conformation  of  the  skeleton  is  adapted  to  this  new 
and  important  function.  The  chest  is  broad  and  capacious 
to  admit  of  free  respiration  while  the  animal  is  flying,  and 
to  afi'ord  ample  space  for  the  attachment  of  the  large  mus- 
cles which  have  become  necessary.  The  scapulae  (s)  are 
large,  and  of  a  singular  form,  and  they  are  kept  at  a  consi- 
derable distance  asunder  by  the  expanded  chest:  their  cora- 
coid  processes  are  also  large,  and  extend  in  the  direction  of 
the  sternum.  The  clavicles  (c)  are  of  enormous  size  and 
length,  being  larger  than  either  the  scapula  or  the  sternum, 
and  remarkably  curved  in  their  shape.  The  sternum  is 
much  developed,  extending  laterally,  and  having  a  project- 
ing crest  along  the  middle  of  its  lower  surface.  The  hu- 
merus (h)  is  strong,  but  short;  apparently  in  order  to  avoid 
the  danger  of  its  being  snapped  asunder  by  the  violent  ac- 
tions of  the  pectoral  muscles,  had  it  been  longer.  As  the 
leading  object  of  the  structure  is  to  give  power  to  the  wing, 
there  was  no  necessity  for  the  rotatory  motion  of  the  bones 
of  the  fore-arm;  and  accordingly  we  find  them  consolidated 
into  one  (r;)  or  rather  no  part  of  the  ulna  is  developed,  ex- 
cept the  process  of  the  olecranon,  or  elbow,  which  has  be- 
come soldered  to  the  radius. 

These  advantages  in  the  construction  of  the  fore  extremi- 
ties are  obtained  at  the  expense  of  the  hinder,  which  are  too 
feeble  to  support  the  weight  of  the  body  in  the  upright  posi- 
tion required  for  walking,  in  consequence  of  the  centre  of  gra- 
vity being  between  the  wings.  On  a  level  plane,  indeed,  the 
bat  can  advance  only  by  a  kind  of  crawling  or  hoj^ping  motion. 
The  whole  anterior  half  of  the  trunk  is  much  more  fully  de- 
veloped than  the  posterior  half,  which  appears  as  if  it  had 
been  checked  in  its  growth.  The  pelvis  (p)  is  of  diminutive 
size,  compared  with  the  rest  of  the  skeleton :  the  pubic  bones 
are  lengthened  backwards,  and  are  joined  merely  at  a  small 
point.     The  whole  posterior  limb  is  short,  the  femur  (f )  com- 


382  THE  MECHANICAL  FUNCTIONS. 

paratively  long,  and  the  fibula  is  a  very  slender  bone,  yet 
quite  distinct  from  the  tibia  (t.)  The  ^slight  degree  of  mo- 
tion which  is  thus  allowed  between  them  is  useful  to  the  ani- 
mal, in  enabling  the  feet  to  lay  hold  of  cornices  or  other  pro- 
jecting parts  of  the  roofs  of  buildings,  on  which  the  animal 
fastens  itself,  and  hangs  with  the  head  downwards.  It  is 
probably  with  the  intention  of  facilitating  this  action  that  the 
toes  are  turned  completely  backwards;  and  that  they  are  of 
a  curved  shape,  and  generally  armed  with  sharp  claws.  A 
bony  appendix  (a)  projects  outwards  from  the  heel,  for  the 
purpose  of  supporting  the  hinder  prolongation  of  the  mem- 
brane, which  often  extends  between  the  hind  feet,  and  is  far- 
ther sustained  by  the  tail,  in  those  species  which  have  the 
spine  prolonged  to  form  one. 

Bats  are  also  provided  with  another  instrument  for  sus- 
pending themselves  to  projecting  objects,  formed  by  the 
thumb  (b,)  which  is,  apparently  for  this  express  purpose, 
detached  from  the  fingers  that  support  the  wing,  and  is  ter-, 
minated  by  a  strong  claw,  which  projects,  even  when  the 
wings  are  folded,  and  is  useful  in  progression,  serving  as  a 
point  of  support. 

§  2.  Birds. 

It  is  in  birds  alone  that  we  find  the  most  perfect  adapta- 
tion of  structure  to  the  purposes  of  rapid  and  extensive 
flight:  in  them  the  frame  of  the  skeleton,  the  figure,  position, 
and  structure  of  the  wings,  the  size  of  the  muscles,  the  pecu- 
liar nature  of  their  irritability,  and  even  the  outward  form 
of  the  body,  have  all  a  direct  and  beautiful  relation  to  the 
properties  of  the  element  in  which  nature  has  intended  them 
to  move.  In  their  formation,  a  new,  and  in  as  far  as  relates 
to  the  organs  of  progressive  motion,  a  more  developed  type 
is  adopted;  still  preserving  a  conformity  with  the  general 
plan  of  the  vertebral  organization,  and  with  the  general 
laws  of  its  development. 

The  skeleton  of  birds  has  the  same  constituent  parts  as 
that  of  other  vertebrated  classes:  the  bones  of  the  anterior 


BIRDS.  38 


rt 


extremity,  though  destined  exclusively  to  support  the  wing, 
retain  the  same  divisions,  and  are  composed  of  the  usual 
elements:  and  the  general  form  of  the  body  is  that  best  cal- 
culated to  glide  through  the  air  with  the  least  resistance. 
As  birds  swallow  their  food  entire,  there  is  no  necessity  for 
any  part  of  the  bulky  apparatus  of  hard  and  solid  teeth,  large 
muscles  and  heavy  jaws  which  are  required  by  most  quad- 
rupeds: hence  the  head  admits  of  being  greatly  reduced  in 
its,  dimensions;  and  the  form  of  the  beak,  which  is  drawn  to 
a  point,  and  cuts  the  opposing  air,  tends  to  facilitate  the  pro- 
gress of  the  bird  in  its  flight. 

In  the  conformation  of  the  body,  also,  every  circumstance 
that  could  contribute  to  give  it  lightness  has  been  sedulously 
studied.  The  general  size  of  birds  is  considerably  smaller 
than  quadrupeds  of  corresponding  habits.  No  where  has 
Nature  attempted  to  endow  a  huge  ponderous  animal,  like 
the  fabled  Pegasus,  with  the  power  of  flight.  Great  con- 
densation has  been  given  to  the  osseous  substance,'^'  in  order 
that  the  greatest  degree  of  strength  might  be  procured  with 
the  same  weight  of  solid  materials;  and  the  mechanical  ad- 
vantage derived  from  their  being  disposed  in  the  circum- 
ference rather  than  in  central  masses,  has  been  obtained  to 
the  utmost  extent.  The  horny  material,  of  which  the  stems 
of  the  feathers  are  constructed,  arc,  in  like  manner,  formed 
into  hollow  cylinders,  which,  compared  with  their  weight, 
are  exceedingly  strong.  A  similar  shape  has  been  given  to 
the  cylindrical  bones,  which  are  fashioned  into  tubes  with 
dense  but  thin  sides:  most  of  the  other  bones  have  likewise 
been  made  hollow,  and  instead  of  their  cavities  being  filled 
with  marrow,  they  contain  only  air.t  Thus,  the  whole  ske- 
leton is  rendered  remarkably  light:  that  of  the  Pelicanus 

*  Ossification  not  only  proceeds  more  rapidly,  bat  is  also  carried  to  a 
greater  extent  in  this  class  of  animals  than  in  any  other;  as  a  proof  of  which, 
the  tendons,  especially  those  of  the  muscles  of  the  legs,  are  frequently  ossi- 
fied. 

•j-  In  the  bat  there  is  no  provision  of  this  kind  for  lightening  the  boneSy 
and  we  find  them  containing  marrow,  as  in  other  mammalia,  and  not  air. 


384  THE  MECHANICAL  FUNCTIONS. 

07iocrotalus,  for  instance,  or  white  Pelican,  which  is  five 
feet  in  length,  was  found  by  the  Parisian  Academicians  to 
weigh  only  twenty-three  ounces,  while  the  entire  bird 
weighed  nearly  twenty-five  pounds.  The  cavities  in  the 
bones  communicate  with  large  air  cells,  which  are  distri- 
buted in  various  parts  of  the  body,  and  which  contribute 
still  farther  to  diminish  its  specific  gravity:  and  by  means 
of  canals  which  open  into  the  air  passages  of  the  lungs,  this 
air  finds  a  ready  outlet  when  it  becomes  rarefied  by  the  as- 
cent of  the  bird  into  the  higher  regions  of  the  atmosphere.* 
The  conditions  in  which  a  bird  is  placed  with  regard  to 
the  density  of  the  surrounding  medium,  as  well  as  their  mode 
of  progression,  are  so  opposite  to  those  of  fishes,  that  we 
should  expect  to  find  great  corresponding  differences  in 
their  conformation.  These  two  classes  of  vertebrata,  accord- 
ingly, are  remarkably  contrasted  with  respect  to  the  struc- 
ture of  their  skeletons.  In  fishes  we  have  seen  that  the  chest 
and  all  the  viscera  are  carried  as  far  forwards  as  possible; 
the  respiratory  organs  and  the  centre  of  circulation  being 
close  to  the  head,  the  neck  having  disappeared,  and  the 
trunk  being  continued  into  the  lengthened  tail,  in  which  the 
chief  bulk  of  the  muscles  are  situated.     In  birds,  on  the  con- 

*  This  air,  being"  contained  in  the  interior  of  the  body,  which  preserves 
a  very  elevated  temperature,  must  be  constantly  in  a  state  of  greater  rarefac- 
tion than  the  cooler  external  air;  a  condition  which  must  contribute  in  some 
slight  degree  to  render  the  whole  body  lighter  than  it  would  otherwise  have 
been.  It  appears  to  me,  however,  that  considerably  greater  importance  has 
been  attached  to  this  circumstance  than  it  really  possesses.  Many  have  gone 
so  far  as  to  represent  the  condition  of  a  bird  as  approaching  to  that  of  a  bal- 
loon filled  with  a  lighter  gas  than  atmospheric  air:  and  have  been  lavish  in 
their  expressions  of  admiration  at  the  beauty  of  the  contrivance  which  thus 
converted  a  living  structure  into  an  aerostatic  machine.  A  little  sober  con- 
sideration will  suffice  to  show  that  the  amount  of  the  supposed  advantages 
resulting  to  the  bird  from  the  diminution  of  weight,  occasioned  by  the  dif- 
ference of  temperature  between  the  air  included  in  its  body  and  the  exter- 
nal atmosphere,  is  perfectly  insignificant.  Any  one  who  will  take  the  trou- 
ble to  calculate  the  real  diminution  of  weight  arising  from  this  cause,  under 
the  most  favourable  circumstances,  will  find  that,  even  in  the  case  of  the 
largest  bird,  it  can  never  amount  to  more  than  a  few  grains. 


BIRDS. 


'3S5 


trary,  the  ribs,  and  the  viscera  which  they  protect,  are 
placed  as  far  back  along  the  spinal  column  as  possible;  and 
a  long  and  flexible  neck  extends  from  the  trunk  to  the  head, 
which  is  thus  carried  considerably  forwards.  These  circum- 
stances are  very  apparent  in  the  skeleton  of  the  swan,  re- 
presented in  Fig.  224.     In  the  fish,  progressive  motion  is 


eff'ected  principally  by  the  movements  of  the  tail,  which  im- 
pels the  body  alternately  from  side  to  side:  in  the  bird,  the 
only  instruments  of  motion  are  the  wings,  which  are  affixed 
to  the  fore  part  of  the  trunk,  and  are  moved  by  muscles  situ- 
ated  in  that  region.  In  the  fish,  the  spine  is  flexible  ncar- 
VoL.  I.  49 


3S6  THE  MECHANICAL  FUNCTIONS. 

ly  throughout  its  whole  extent;  in  the  bird,  it  is  rigid  and 
immoveable  in  the  trunk,  and  is  capable  of  extensive  motion 
only  in  the  neck. 

In  order  that  the  body  may  be  exactly  balanced  while  the 
bird  is  flying,  its  centre  of  gravity  must  be  brought  precisely 
under  the  line  connecting  the  articulations  of  the  wings  wdth 
the  trunk;  for  it  is  at  these  points  that  the  resistance  of  the 
air  causes  it  to  be  supported  by  the  wings.  When  the  bird 
is  resting  upon  its  legs,  the  centre  of  gravity  must,  in  like 
manner,  be  brought  immediately  over  the  base  of  support 
formed  by  the  toes:  it  becomes  necessary,  therefore,  to 
provide  means  for  shifting  the  centre  of  gravity  from  one 
place  to  another,  according  to  circumstances,  and  to  adjust 
its  position  with  considerable  nicety;  otherwise  there  would 
be  danger  of  the  equilibrium  being  destroyed,  and  the  body 
oversetting.  The  principal  means  of  effecting  these  adjust- 
ments consist  in  the  motions  of  the  head  and  neck,  which  last 
is,  for  that  purpose,  rendered  exceedingly  long  and  flexible. 
The  number  of  cervical  vertebrae  is  generally  very  consi- 
derable: in  the  mammialia,as  we  have  seen,  there  are  always 
seven,  but  in  many  birds  there  are  more  than  twice  that 
number.  In  the  swan  (Fig.  224,)  there  are  twenty-three, 
and  they  are  joined  together  by  articulations,  generally  al- 
lowing free  motion  in  all  directions;  that  is,  laterally,  as  well 
as  forw^ards  and  backwards.  This  unusual  degree  of  mobi- 
lity is  conferred  by  a  peculiar  mechanism,  w^hich  is  not  met 
with  in  the  other  classes  of  vertebrated  animals.  A  cartilage 
is  interposed  between  each  of  the  vertebrae,  to  the  surfaces  of 
which  these  cartilages  are  curiously  adapted,  being  enclosed 
between  folds  of  the  membrane  lining  the  joint:  so  that  each 
joint  is  in  reality  double,  consisting  of  two  cavities,  with  an 
intervening  cartilage.* 

It  is  to  be  observed,  how^ever,  that  in  consequence  of  the 
positions  of  the  oblique  processes,  the  upper  vertebr  aeof 

*  See  Mr.  H.  Earle's  paper  on  this  subject  in  the  Philosophical  transac- 
tions for  1833,  p.  277. 


BIRDS.  387 

the  neck  bend  with  more  facility  forwards  than  backwards; 
while  those  in  the  lower  half  of  the  neck  bend  more  rcacfily 
backwards:  hence,  in  a  state  of  repose,  the  neck  naturally  as- 
sumes a  double  curvature,  like  that  of  the  letter  S,  as  is  well 
seen  in  the  graceful  form  of  the  swan's  neck.  By  extend- 
ing the  neck  in  a  straight  line,  the  bird  can,  while  flying, 
carry  forwards  the  centre  of  gravity,  so  as  to  bring  it  under 
the  wings;  and  when  resting  on  its  feet,  or  floating  on  the 
water,  it  can  transfer  that  centre  backwards,  so  as  to  bring  it 
towards  the  middle  of  the  body,  by  merely  bending  back  the 
neck  into  the  curved  form  which  has  just  been  described;  and 
thus  the  equilibrium  is,  under  all  circumstances  preserved,  by 
movements  remarkable  for  their  elegance  and  grace.* 

Another  advantage  arising  from  the  length  and  mobility 
of  the  neck  is,  that  it  facilitates  the  application  of  the  head 
to  every  part  of  the  surface  of  the  body.  Birds  require  this 
power  in  order  that  they  may  be  enabled  to  adjust  their 
plumage,  whenever  it  has,  by  any  accident,  become  ruffled. 
In  aquatic  birds,  it  is  necessary  that  every  feather  should  be 
constantly  anointed  with  an  oily  secretion,  which  preserves 
it  from  being  wetted,  and  which  is  copiously  provided  for 
that  purpose  by  glands  situated  near  the  tail.  The  flexibili- 
ty of  the  neck  alone  would  have  been  insufficient  for  enabling 
the  bird  to  bring  its  bill  in  contact  with  every  feather,  in 
order  to  distribute  this  fluid  equally  over  them;  and  there  is, 
accordingly,  a  farther  provision  made  for  the  accomplish- 
ment of  this  object  in  the  mode  of  articulation  of  the  head 
with  the  neck.  We  have  seen  that,  in  fishes,  and  in  most 
reptiles,  this  articulation  consists  of  a  ball  and  socket  joint; 
a  rounded  tubercle  of  the  occipital  bone  being  received  into 
a  hemispherical  depression  in  the  first  vertebra  of  the  neck. 
In  the  mammalia  the  plan  is  changed,  and  there  are  two  ar- 

*  The  great  mobility  of  the  neck  enables  the  bird  to  employ  its  beak  as 
an  organ  of  prehension  for  taking- its  food:  an  object  which  was  the  more 
necessary  in  consequence  of  the  conversion  of  the  fore  extremities  into  wing^ 
of  which  the  structure  is  incompatible  with  any  prehensile  power,  such  as  is 
often  possessed  by  the  anterior  extremity  of  a  quadruped. 


383 


THE  MECHANICAL  EUNCTIONS. 


ticular  surfaces,  one  on  each  side  of  the  spinal  canal,  formed 
on' processes  corresponding  to  the  leaves  of  the  first  cranial 
vertebra,  and  assimilating  it  more  to  a  hinge  joint.  In  birds, 
however,  where,  as  we  have  just  seen,  the  most  extensive 
lateral  motions  are  required,  the  plan  of  the  ball  and  socket 
joint  is  again  resorted  to;  and  the  occipital  bone  is  made  to 
turn  upon  the  atlas  by  a  single  pivot.  So  great  is  the  free- 
dom of  motion  in  this  joint,  that  the  bird  can  readily  turn 
its  head  completely  back  upon  its  neck,  on  either  side. 

As  spinous  or  transverse  processes  of  any  length  would 
have  interfered  with  the  flexions  of  the  neck,  we  find  scarce- 
ly a  trace  of  these  processes  in  the  cervical  vertebrae  of  birds. 
But  another,  and  a  still  more  important  consideration  was  to 
be  attended  to  in  the  construction  of  this  part  of  the  spine. 
It  must  be  recollected  that  the  spinal  marrow  passes  down 
along  the  canal  formed  by  the  arches  of  the  vertebrae,  and 
that  any  pressure  applied  to  its  tender  substance  would  in- 
stantly paralyze  the  w^hole  body,  and  speedily  put  an  end  to 
life.  Some  extraordinary  provision  was  therefore  required 
to  be  made,  in  order  to  guard  against  the  possibility  of  this 
accident  occurring  during  the  many  violent  contortions  into 
which  the  column  is  liable  to  be  thrown.  This  is  accom- 
plished in  the  simplest  and  most  effectual  manner,  by  en- 
larging the  diameter  of  the  canal  at  the  upper  and  lower  part 
225     ^  of  each  vertebra,  while,  at  the 

middle,  it  remains  of  the  usual 
size,  so  that  the  shape  of  the  ca- 
vity, as  is  well  seen  in  Fig.  225, 
which  shows  a  vertical  section 
of  one  of  the  cervical  vertebrae 
of  the  ostrich,  resembles  that  of 
an  hour  glass.*  Thus,  a  wide 
space  is  left  at  the  junction  of 
each  successive  vertebra,  allow- 
ing of  very  considerable  flexion, 

•  For  the  specimen  from  which  this  engraving  wa,s  made,  I  am  indebted 
to  the  kindness  of  Mr.  Owen. 


BIRDS.  3S9 

without  reducing  the  diameter  of  the  canal  beyond  that  of 
the  narrow  portion,  and,  tlierefore,  without  producing  com- 
pression of  the  spinal  marrow.  Mr.  Earle  found'  that  ver- 
tebrae united  in  this  manner  may  be  bent  backwards  to  a 
right  angle,  and  laterally  to  half  a  right  angle,  without  inju- 
ry to  the  enclosed  nervous  substance.  The  design  of  this 
structure  is  farther  evident  from  its  not  existing  in  the  dor- 
sal and  lumbar  portions  of  the  spine,  which  admit  of  no  mo- 
tion whatever,  and  where  there  is  no  variation  in  the  diame- 
ter of  the  spinal  canal. 

A  plan  entirely  different  is  followed  in  the  vertebrse  of 
the  back  and  loins.  For  the  purpose  of  ensuring  the  proper 
actions  of  the  wings,  the  great  object  here  is  to  prevent  mo- 
tion, and  to  give  all  possible  strength  and  security;  and  ac- 
cordingly the  whole  of  this  portion  of  the  spine,  together 
with  the  sacrum,  is  consolidated  into  one  piece.  All  the 
processes  are  largely  developed,  and  pass  obliquely  from 
one  vertebra  to  the  next,  mutually  locking  them  together: 
and,  in  order  most  effectually  to  preclude  the  possibility  of 
any  flexion,  the  spinous  processes,  and  sometimes  even  tl:;^^ 
bodies  of  the  dorsal  vertebrae  are  immoveably  soldered  to- 
gether by  ossific  matter,  so  as  to  form  one  continuous  bone. 

The  sacrum  (v,  Fig.  224)  consists  of  the  union  of  a  great 
number  of  vertebrae,  as  many  as  twenty  being  anchylosed 
together  for  this  purpose;  so  that  they  form  a  bone  of  great 
length.  The  coccygeal  vertebrae  (q)  are  also  numerous,  but 
are  compressed  into  a  small  space,  and  enjoy  great  latitude 
of  motion,  being  subservient  to  the  movements  of  the  tail. 

The  ribs  are  numerous,  and  of  considerable  strength:  they 
send  out  processes,  which  are  directed  backwards,  passing 
over  the  next  rib  before  they  terminate,  and  giving  very  ef- 
fectual support  to  the  walls  of  the  chest.  The  ribs  are  con- 
tinued along  the  abdomen,  and  afford  protection  to  the  vis- 
cera in  that  cavity;  and  some  arise  even  from  the  sacrum, 
and  from  the  iliac  bones.     Those  which  are  in  front  are 

*  In  the  paper  already  quoted,  p.  278. 


390  THE  MECHANICAL  FUNCTIONS. 

united  to  the  sternum  (s)  by  means  of  sternal  appendices, 
which  are  ossified,  and  appear  as  the  continuations  of  the 
ribs,  or  as  if  the  ribs  were  jointed  in  the  middle. 

The  sternum  is  of  enormous  size,  extending  over  a  con- 
siderable part  of  the  abdomen,  and  having  a  large  perpendi- 
cular crest  descending,  like  the  keel  of  a  ship,  from  its  lower 
surface.  The  object  of  this  great  development  is  to  furnish 
extensive  attachment  to  the  large  pectoral  muscles  employed 
to  move  the  wings,  and  which,  taken  together,  are  generally 
heavier  than  the  rest  of  the  body.  Considered  with  refe- 
rence to  all  the  other  muscles,  and  to  the  weight  of  the  body 
itself,  these  pectoral  muscles  are  of  enormous  strength.  The 
flap  of  a  swan's  wing  is  capable  of  breaking  a  man's  leg;  and 
a  similar  blow  from  an  eagle  has  been  known  to  be  instantly 
fatal.  The  bat  is  the  only  instance,  among  the  mammalia, 
where  the  sternum  presents  this  peculiar  carinated^  or  keel- 
like shape:  and  the  purpose  is  evidently  the  same  as  in  the 
bird.* 

The  scapula  is  generally  a  small  and  slender  bone.  The 
Jfcpracoid  bone  (k)  is  largely  developed,  and  assumes  much  of 
the  appearance  of  a  clavicle.t  But  the  real  clavicles  (c)  are 
united  below,  where  they  join  the  fore  part  of  the  sternum, 
appearing  as  one  bone,  which,  from  its  forked  shape,  has 
been  denominated  the  furcidar  bone.  In  the  fowl  it  is 
commonly  known  by  the  name  of  the  merry -thought.  This 
bone,  placed  at  the  origin  of  the  wings,  and  stretching  from 
one  to  the  other,  is  of  great  importance  as  constituting  a  firm 
basis  for  their  support,  and  for  securing  their  steadiness  of 
action;  and  being,  at  the  same  time,  very  elastic,  it  tends  to 

*  Notwithstanding  the  great  modification  the  sternum  has  received  in  the 
bird,  when  compared  with  its  form  in  the  tortoise  and  the  quadruped,  we 
may  still  trace  the  same  nine  elements  entering"  into  its  composition,  though 
developed  in  very  different  proportions. 

\  Many  have  considered  this  bone  as  being  the  clavicle,  and  have  regarded 
the  furcular  bone  as  a  new  bone,  or  supplementary  clavicle:  but  all  the  ana- 
logies of  position  and  of  development  are  in  favour  of  the  views  stated  in  the 
text. 


BIRDS.  391 

restore  them  to  their  proper  situations,  after  they  have  been 
disturbed  by  any  violent  impulse. 

The  wing  of  a  bird  does  not,  at  first  view,  present  much 
analogy  with  the  fore  extremity  of  a  quadruped:  but  on  a 
closer  examination  we  find  it  to  contain  all  the  principal 
bones  of  the  latter,  though  somewhat  altered  in  shape,  and 
still  more  changed  in  their  functions.  Yet  still  the  same 
unity  of  plan,  and  perfect  harmony  of  execution  may  be 
discerned  in  the  mechanism  of  this  refined  instrument  of  a 
higher  mode  of  progression. 

The  head  of  the  humerus  (h)  has  a  compressed  form;  and 
in  order  to  obtain  great  extent  of  motion,  it  is  made  to  play 
by  a  very  small  cylindrical  surface  upon  the  scapula;  thus 
admitting  of  the  complete  descent  of  the  wing,  unobstructed 
by  any  opposing  process,  but  at  the  same  time  limiting  its 
motion  to  one  plane.     It  is  connected  below,  by  broad  at- 
tachm.ents,  to  the  radius  and  ulna,  forming  with  them  a  hinge 
joint.     These  latter  bones  are  separate,  and  of  great  length, 
but  so  firmly  united  together  by  ligament  as  scarcely  to  have 
any  motion  on  one  another.     The  carpus  (w,)  consists  of 
two  bones  only,  the  one  articulated  with  the  radius,  the 
other  with  the  ulna.     They  move  together  as  one  piece; 
but,  contrary  to  what  takes  place  in  quadrupeds,  the  move- 
ments are  made  from  side  to  side,  instead  of  their  consistino- 
of  flexion  and  extension;  this  variation  from  the  usual  struc- 
ture being  for  the  purpose  of  folding  down  the  joints  of  the 
wing,  and  bringing  them  close  to  the  bod}^    .The  metacar- 
pus (m)  consists  originally  of  two  bones,  which  soon  become 
united  into  one  at  the  upper  part.     On  the  radial  side  it  has 
a  process,  derived  perhaps  from  a  third  metacarpal  bone, 
which  is  anchylosed  at  a  still  earlier  period  of  ossification; 
and  to  this  process  a  small  pointed  bone  is  connected,  cor- 
responding to  a  rudimental  thumb  (t.)     There  are  generally 
two  fingers,  of  which  the  first  exhibits  traces  of  having  been 
originally  two  bones:  the  inner  finger  consists  of  two  or 
three  long  phalanges,  and  the  outer  one  of  a  single  phalanx: 
there  is  sometimes  also  a  rudimental  bone  corresponding  to 


392  THE  MECHANICAL  FUNCTIONS. 

a  little  finger.     The  degree  of  development  of  these  bones 
varies  in  different  tribes  of  birds. 

Feathers  are  attached  to  all  these  divisions  of  the  limb, 
namely,  to  the  humerus,  the  fore  arm,  the  hand,  and  occa- 
sionally to  the  single  phalanx  of  the  thumb.     The  structure 
of  feathers  is  calculated  in  an  eminent  degree  to  combine 
the  qualities  of  lightness  and  of  strength,  which  we  else- 
where rarely  find  united.     The  horny  materials  of  which 
the  stem  of  the  quill  is  made  are  tough,  pliant,  and  elastic; 
and,  as  we  have  already  seen,  are  disposed  in  the  most  ad- 
vantageous manner  for  resisting  flexion  by  being  formed 
into  a  hollow  cylinder.     But  the  vane  of  the  feather  is  still 
more  artificially  constructed;  being  composed  of  a  number 
of  flat  threads,  or  filaments,  so  arranged  as  to  oppose  a  much 
greater  resistance  to  a  force  striking  perpendicularly  against 
their  surface,  than  to  one  tvhich  is  directed  laterally;  that 
is,  in  the  plane  of  the  stem.     They  derive  this  power  of  re- 
sistance from  their  flattened  shape,  which  allows  them  to 
bend  less  easily  in  the  direction  of  their  flat  surfaces  than 
in  any  other;  in  the  same  way  that  a  slip  of  card  cannot 
easily  be  bent  by  a  force  acting  in  its  own  plane,  though  it 
easily  yields  to  one  at  right  angles  to  it.     Now  it  is  exactly 
in  the  direction  in  which  they  do  not  bend  that  the  fila- 
ments of  the  feather  have  to  encounter  the  resistance  and 
impulse  of  the  air.     It  is  here  that  strength  is  wanted,  and 
it  is  here  that  strength  has  been  bestowed. 

On  examining  the  assemblage  of  these  laminated  filaments 
still  more  minutely,  we  find  that  they  appear  to  adhere  to 
one  another.  As  we  cannot  perceive  that  they  are  united 
by  any  glutinous  matter,  it  is  evident  that  their  connexion 
must  be  effected  by  some  mechanism  invisible  to  the  unas- 
sisted eye.  By  the  aid  of  the  microscope,  the  mystery  is 
unravelled,  and  we  discover  the  presence  of  a  number  of 
minute  fibrils,  arranged  along  the  margin  of  the  laminas,  and 
fitted  to  catch  upon  and  clasp  one  another,  whenever  the  la- 
minae are  brought  w^ithin  a  certain  distance.  The  fibrils  of 
a  feather  from  the  wing  of  a  goose  are  represented  magnified 


FEATHERS  OF  BIRDS. 


393 


at  a,  a,  b,  b,  Fig.  226,  as  they  arise  from  the  two  sides  of 
the  edges  of  each  lamina:  they  are  exceedingly  numerous, 
above  a  thousand  being  contained  in  the  space  of  an  inch; 


and  they  are  of  two  kinds,  each  kind  having  a  different  form 
and  curvature.  Those  marked  a,  a,  which  arise  from  the 
side  next  to  the  extremity  of  the  feather  are  branched  or 
tufted,  and  bend  downwards,  while  those  marked  b,  b,  pro- 
ceeding from  the  other  side  of  the  lamina,  or  that  nearest 
the  root  of  the  feather,  are  shorter  and  firmer,  and  do  not 
divide  into  branches,  but  are  hooked  at  the  extremities,  and 
Ifire  directed  upwards.  When  the  two  laminae  are  brought 
close  to  one  another,  the  long,  curved  fibrils  of  the  one  be- 
ing carried  over  the  short  and  straight  fibrils  of  the  other, 
both  sets  becom.e  entangled  together;  their  crooked  ends 
fastening  into  one  another,  just  as  the  latch  of  a  door  falls 
into  the  cavity  of  the  catch  which  is  fixed  in  the  door-post 
to  receive  it.  The  way  in  which  this  takes  place  will  be 
readily  perceived  by  making  a  section  of  the  vane  of  a  fea- 
ther across  the  lam.ina^,  and  examining,  with  a  good  micro- 
scope, their  cut  edges,  while  they  are  gently  separated  from 
one  another.  The  appearance  they  then  present  is  exhibited 
in  Fig.  227,  which  shows  distinctly  the  form,  direction,  and 
relative  positions  of  each  set  of  fibrils,  and  the  manner  in 
which  they  lay  hold  of  one  another.  This  mechanism  is  re- 
peated over  every  part  of  the  feather,  and  constitutes  a  close- 
VoL.  I.  50 


394  THE  MECHANICAL  FUNCTIONS. 

!y  reticulated  surface  of  great  extent,  admirably  calculated 
to  prevent  the  passage  of  the  air  through  it,  and  to  create,  by 
its  motion,  that  degree  of  resistance  which  it  is  intended  the 
wing  should  encounter.*  In  feathers  not  intended  for  flight, 
as  in  those  of  the  ostrich,  the  fibrils  are  altogether  wanting: 
in  those  of  the  peacock's  tail,  the  fibrils,  though  large,  have 
not  the  construction  which  fits  them  for  clasping  those  of 
the  contiguous  lamina;  and  in  other  instances  they  do  so 
very  imperfectly. 

A  construction  so  refined  and  artificial  as  the  one  I  have 
been  describing,  and  so  perfectly  adapted  to  the  mechanical 
object  which  it  is  toansw^er,  cannot  be  contemplated  without 
the  deepest  feeling  of  admiration,  and  without  the  most  eager 
curiosity  to  gain  an  insight  into  the  elaborate  processes, 
v^^hich,  we  cannot  doubt,  are  employed  by  nature  in  the  for- 
mation of  a  fabric  so  highly  finished,  and  displaying  such 
minute  and  curious  workmanship.  It  is  only  very  recently 
that  we  have  been,  admitted  to  a  close  inspection  of  the  com- 
plicated machinery,  which  is  put  in  action  in  this  branch  of 
what  may  be  called  organic  architecture;  and  certainly  none 
is  more  fitted  to  call  forth  our  profoundest  wonder  at  the 
comprehensiveness  of  the  vast  scheme  of  divine  providence, 
which  extends  its  care  equally  to  the  perfect  construction  o% 
the  minutest  and  apparently  most  insignificant  portions  of 
the  organized  frame,  whether  it  be  the  down  of  a  thistle,  the 
scales  of  a  moth,  or  the  fibrils  of  a  feather,  as  well  as  to 
the  completion  of  the  larger  and  more  important  organs  of 
vitality. 

Every  bird,  on  quitting  the  egg,  is  found  to  be  covered 
on  all  parts  except  the  under  side,  with  a  kind  of  down  con- 
sisting of  minute  filaments,  collected  in  tufts,  and   resem- 

*  A  very  clear  account  of  the  mechanism  described  in  the  text  is  given  by 
Paley,  in  the  12th  chapter  of  his  "Natural  Theology."  Many  of  the  mi- 
nuter details  1  have  supplied  from  my  own  observations  with  the  microscope. 
The  branched  forni  of  the  upper  fibrils,  and  the  reticulated  structure  of  the 
laminx  themselves,  when  viewed  witli  a  high  magnifying  power,  are  parti- 
cularly beautiful  microscopic  objects. 


FEATHERS  OF  BIRDS.  395 

bllng  in  their  arrangement  the  fibres  of  a  camcl-liair  pencil. 
Each  tuft  contains  about  ten  or  twelve  filaments,  growing 
from  the  upper  ends  of  bulbous  roots  implanted  in  the  skin, 
and  which  are  the  rudiments  of  the  organs  that  afterwards 
form  the  feathers,  of  which  this  down,  serving  the  purpose 
of  a  first  garment,  hastily  spread  over  the  young  bird,  is  but 
the  precursor;  for  the  tufts  generally  soon  fall  off  and  disap- 
pear, except  in  the  rapacious  tribes,  as  the  eagle  and  the  vul- 
ture, where  they  remain  attached  to  the  feathers  for  a  consi- 
derable time. 

While  this  temporary  protection  is  given  to  the  integu- 
ment, extensive  preparations  are  making  underneath  for  fur- 
nishing a  more  effective  raiment,  adapted  to  the  future  wants 
of  the  bird.  The  apparatus  by  which  the  feathers  are  to  be 
formed  is  gradually  constructing;  and  its  rudiments  are  re- 
ceiving the  necessary  supply  of  nuti-icnt  juices,  and  of  ves- 
sels for  their  circulation,  together  with  their  usual  comple- 
ment of  nerves  and  absorbents.  When  first  visible,  this 
organ  has  the  form  of  a  very  minute  cone,  attached  by  a  fila- 
ment proceeding  from  its  base  to  one  of  the  papillae  of  the 
skin,  and  establishing  its  connexion  with  the  living  system. 
In  the  course  of  a  few  days,  this  cone  has  become  elongated 
into  a  cylinder,  with  a  pointed  extremity,  while  its  base  is 
united  to  the  skin  by  a  more  distinct  bond  of  connexion 
formed  by  the  enlarged  vessels,  which  are  supplying  it  with 
nourishment.  It  is  in  the  interior  of  this  cylinder  that  all 
the  parts  of  the  feather  are  constructed;  their  earliest  rudi- 
ments being  formed  at  the  upper  part,  or  apex  of  this  organ; 
and  the  materials  of  the  several  parts  of  the  feather  being 
successively  deposited  and  fashioned  into  their  j)roper  shapes 
in  different  places:  for  while  the  first  lamina?  are  construct- 
ing in  one  portion  of  the  cylinder,  the  next  are  only  just  be- 
^innins;  to  be  formed  in  another;  and  while  the  outer  covcr- 
ing  of  the  stem  is  growing  from  one  membrane,  the  interior 
spongy  tissue  is  dejiosited  in  other  places,  in  various  stages 
of  softness  or  consolidation:  so  that  the  whole  comj)oses  a 
system  of  operations,  which  may  be  said  to  resemble  in  its 


396 


THE  MECHANICAL  FUNCTIONS. 


complication  at  least,  although  on  a  microscopic  scale,  an 
extensive  manufactory.  Hence  will  be  readily  understood 
how  great  must  be  the  difficulty  of  tracing  all  the  steps  of 
these  multifarious  processes,  which  are  carried  on  in  so 
small  a  space:  and  this  difficulty  is  much  increased  from  the 
circumstance  that  the  organ  in  which  they  take  place  is  it- 
self only  developed  as  the  work  proceeds,  its  different  parts 
being  produced  successively  in  proportion  as  they  are  want- 
ed, and  their  form  and  structure  undergoing  frequent  varia- 
tion in  the  course  of  their  development. 

230  231 


228      .-  229 

i-!'!"il)llllK  H  A 


S-^- 


nriiiii!. 


The  most  elaborate  and  apparently  accurate  researches  on 
this  intricate  subject,  are  those  lately  undertaken  by  M. 
Frederick  Cuvier,  from  whose  memoir*  I  have  selected 
the  following  abridged  statement  of  the  principal  results  of 
his  observations.     It  will  be  necessary,  in  order  to  obtain  a 

*  M^moires  du  Museum,  xiii.  327;  and  Annales  des  Sciences  Nuturelles, 
ix.  113. 


FEATHERS  OF  BIRDS,  397 

clear  idea  of  the  several  steps  of  the  process  to  be  described, 
to  advert  to  the  structure  of  a  feather  in  its  fiiiislicd  state. 
For  this  purpose  we  need  only  examine  a  common  feather, 
such  as  that  represented  in  Fig.  228,  where  s  is  the  posterior 
surface  of  the  solid  stem,  which,  it  will  be  perceived,  is 
divided  into  two  parts  by  a  longitudinal  groove,  and  from 
either  side  of  which  proceed  a  scries  of  laminae,  composing, 
with  their  fibrils,  what  is  termed  the  vane  of  the  feather 
(v.)  The  lines  from  which  these  laminsc  arise,  approach 
one  another  at  the  lower  part  of  the  stem,  till  they  meet  at 
a  point,  where  the  longitudinal  groove  terminates,  and  where 
there  is  a  small  orifice  (o,)  leading  to  the  interior  of  the 
quill.  From  this  part  the  transparent  tubular  portion  of  the 
quill  (t)  commences;  and  at  its  lower  extremity  (l)  there 
exists  a  second,  or  lower  orifice. 

The  entire  organ  which  forms  the  feather,  and  which  may 
be  termed  its  matrix,  is  represented  in  Fig.  229,  when  it 
has  attained  the  cylindric  form  already  described;  of  which 
A  is  the  apex,  or  conical  part,  that  rises  above  the  cuticle, 
and  B  the  base,  by  which  it  is  attached  to  the  cerium,  or  true 
skin.  A  white  line  is  seen  running  longitudinally  the  whole 
length  of  the  cylinder,  and  anotlier,  exactly  similar  to  it,  is  ^ 
met  with  on  the  opposite  side:  the  one  corresponds  in  situ- 
ation to  the  front,  and  the  otlicr  to  the  back  o^the  stem  of 
the  future  feather.  On  laying  open  the  matrix  longitudi- 
nally, as  is  shown  in  Fig.  230,  it  is  found  to  be  composed 
of  a  sheath  or  capsule,  and  of  a  central  pulpy  mass,  termed 
the  hulh.  The  capsule  consists  of  several  membranous  lay- 
ers (c,  E,  s,  I,)  which  are  more  consolidated  near  the  apex, 
and  become  gradually  softer  and  more  delicate,  as  we  trace 
them  towards  the  base  of  the  matrix,  where  their  formation 
is  only  beginning  to  take  place. 

The  laminre  and  theiV  fibrils,  the  assemblage  of  whicli 
constitutes  the  vane  of  the  feather,  arc  tlic  parts  which  arc 
first  formed;  and  their  construction  is  cflected  in  the  sjiace 
between  the  outer  capsule  (c,)  and  the  centKil  bulb  (n,)  in  a 
mode  which  is  exceedingly  remarkable,  and  dillcrcnt  from 


39S  THE  MECHANICAL  FUNCTIONS. 

that  of  the  formation  of  any  other  organic  product  with 
which  we  are  acquainted.  Instead  of  growing  from  a  base, 
like  hairs,  and  other  productions  of  the  integuments,  by  suc- 
cessive depositions  of  layers,  the  materials  which  are  to 
compose  the  laminae  are  cast  in  moulds,  where  they  harden 
and  acquire  the  exact  shape  of  the  recipient  ca^aties.  The 
next  object  of  our  curiosity,  then,  is  to  learn  the  way  in 
which  these  moulds  are  constructed;  and  on  careful  exami- 
nation they  appear  to  be  formed  by  two  striated  membranes, 
the  exterior  one  (e)  enveloping  the  other  (i,)  or  interior 
membrane.  These  membranes  are  separated  by  a  series  of 
partitions,  which  commence  at  the  edges  of  the  longitudinal 
white  band,  seen  in  Fig.  229,  and  wind  obliquely  upwards 
till  they  reach  the  opposite  longitudinal  band  already  de- 
scribed, where  they  join  a  longitudinal  partition  which  oc- 
cupies a  line  answering  to  that  posterior  band.  Thus  they 
leave  between  them  narrow  spaces,  which  constitute  so 
many  compartments  for  the  deposition,  as  in  a  mould,  of  the 
material  of  each  lamina.  The  course  of  these  channels,  and 
their  junction  at  the  back  of  the  matrix  is  seen  at  s,  Fig. 
230.  It  is  exceedingly  probable,  though  from  the  minute- 
ness of  the  parts  it  is  scarcely  possible  to  obtain  ocular  de- 
monstration of  the  fact,  that  the  fibrils  of  the  laminae  are 
formed  in  a  similar  manner,  by  being  moulded  in  still  more 
minute  compartments,  formed  by  transverse  membranous 
partitions. 

The  proper  office  of  the  bulb,  after  it  has  supplied  the 
materials  for  the  formation  of  the  laminae,  is  to  construct 
the  stem  of  the  feather,  and  unite  the  laminae  to  its  sides. 
For  this  purpose  the  anterior  portion  of  the  bulb  deposites 
on  its  surface  a  plate  of  horny  substance,  while  another  plate 
is  formed  by  the  posterior  part  in  the  interior  of  the  bulb. 
Thus  the  bulb  becomes  divided  into  two  portions,  one  ante- 
rior and  the  other  posterior.  The  former  of  these,  after 
having  finished  the  external  plate,  proceeds  to  form  the 
spongy  substance,  which  is  to  connect  the  two  plates,  and 
the  posterior  portion  of  the  bulb  embraces  the  inner  plate, 


i, 
FEATHERS  OP  BIRDS.  399 

and  gradually  folds  it  inwards  till  its  sides  meet  at  the  mid- 
dle groove  along  the  back  of  the  stem.  The  anterior  part 
of  the  bulb,  during  the  process  of  fdling  up  the  stem,  exhi- 
bibits  a  series  of  conical  shaped  membranes,  as  is  seen  in 
the  section,  Fig.  231;  the  points  of  the  cones  being  directed 
upwards,  and  their  intervals  being  occupied  hy  the  spongy 
substance  in  different  stages  of  consolidation,  and  more  per- 
fected in  proportion  as  they  are  situated  nearer  the  apex  of 
the  stem. 

While  the  construction   of  tlie  feather,  in   Its  different 
stages,   is  thus  advancing  from  below,  those  parts    which 
are  completely  formed,  are  rising  above  the  surAice  of  the 
skin, still  enveloped  in  the  capsule  which  originally  protected 
them,  but  the  upper  portions  of  which,  from  the  action  of 
the  air,  and  the  obliteration  of  the  vessels  that  nourished 
them,  now  decaying,  shrivel  and  fall  off  in  shreds,  allowing 
the  successive  portions  of  the  feather  to  come  forth,  and  the 
lamincE  to  unfold  themselves  as  they  rise  and  assume  their 
proper-  shapes.     This  successive  evolution  proceeds  until  the 
principal  parts  of  the  stem  and  of  the  vane  are  completed; 
and  then  a  different" kind  of  action  takes  place.     The  poste- 
rior part  of  the  bulb  now  contracts  itself,  and   brino-ino;  the 
edges  of  that  surf^ice  of  the  stem  closer  toiiether,  at  lenirth 
unites  them  at  the  superior  orifice  (o,)  Fig.  22S;  where  the 
laminae,  which  follow  these  lines,  also  terminate.     Having 
thus  performed    the   office  assigned  to  it,  it  ceases  to  be 
nourished,  and  is  incapable  any  longer  of  depositing  a  horny 
covering  to  the  feather:  all  that  remains  of  its  substance  is  a 
thin  membrane  which  adheres  to  the  outside  of  the  tubular 
part  or  barrel  of  the  quill,  and  which  must  be  scraped  off 
before  the  latter  can  be  used  as  a  pen.     The  tubular  part  is 
the  product  of  the  anterior  part  of  the  bulb,  which   now 
ceases  to  deposite  the  spongy  substance,  but  forms  a  transpa- 
rent horny  material  over  the  whole  of  its  external  surface; 
but  as  it  retires  towards  the  root,  it  leaves  a  succession  of 
very  thin  pellucid  membranes,  in  the  form  of  cones,  which, 
when  dried,  form  what  is  termed  the  pith  of  the  quill.    The 


400  THE  MECHANICAL  FUNCTIONS. 

last  remnant  of  the  bulb  is  seen  in  the  slender  ligament 
which  passes  through  the  lower  orifice,  and  preserves  the 
attachment  of  the  feather  to  the  skin.  In  process  of  time, 
this  also  decays,  and  the  whole  feather  is  cast  off,  preparato- 
ry to  the  formation  of  another,  which,  in  due  season,  is  to 
replace  it.  All  the  feathers  are,  in  general,  moulted  annual- 
ly, or  even  at  shorter  periods;  and  the  same  complicated 
process  is  again  begun  and  completed  by  a  new  matrix  pro- 
duced for  the  occasion,  every  time  a  new  feather  is  to  be 
formed. 

It  is  impossible,  on  reviewing  these  curious  facts,  not  to 
be  struck  with  the  admirable  art  and  foresight  which  are 
implied  in  all  this  long  and  complicated  series  of  operations. 
While  the  bird  was  yet  nourished  by  the  fluids  of  the  egg, 
the  ground  had  already  been  prepared  for  its  future  plumage, 
and  for  the  formation  of  instruments  of  flight.  A  tempora- 
ry investment  of  down  is  in  readiness  to  shelter  the  tender 
chicken  from  the  rude  impressions  of  the  air,  and  an 
apparatus  is  preparing  for  the  construction  of  the  most  re- 
fined instruments  for  clothing  and  for  motion:  first,  the  scaf- 
folding, as  it  may  be  called,  is  erected,  by  the  help  of  which 
each  portion  is  built  up  in  succession,  and  in  proper  order. 
Nature's  next  care  is  to  construct  the  vane,  which  is  the  part 
of  the  feather  most  essential  to  its  office:  and  then  to  form 
the  shaft  to  which  the  vane  is  to  be  affixed,  and  from  which 
it  receives  its  support:  lastly,  she  forms  the  barrel  of  the 
quill,  which  is  prolonged  for  the  purpq^e  of  converting  it 
into  a  lever  of  sufficient  length  for  the  mechanical  office  it 
has  to  perform.  In  proportion  as  each  structure  is  finished, 
she  neglects  not  to  remove  the  scaffolding  which  had  been 
setup  as  a  temporary  structure;  the  membranes,  with  all 
their  partitions,  are  carried  away,  the  vascular  pulp  of  the 
bulb  is  absorbed,  and  its  place  supplied  by  air,  tiius  securing 
the  utmost  lightness,  without  any  diminution  of  strength. 
Is  it  possible  for  any  rational  mind,  after  meditating  upon 
these  facts,  to  arrive  at  the  persuasion  that  they  are  all  the 
mere  results  of  chance? 


WING  OF  BIRDS.  401 

Several  circumstances  remain  to  be  noticed  respecting  the 
structure  and  actions  of  the  wings  of  birds.   If  we  attend  to  the 
mode  of  their  articulation  with  the  scapula,  we  find  it  ])roducing 
a  motion  oblique  with  regard  to  the  axis  of  the  body,  so  that 
the  stroke  which  they  give  to  the  air  is  directed  both  down- 
wards and  backwards;  and  the  bird,  while  moving  forwards, is 
at  the  same  time  supported  in  opposition  to  the  force  of  gra- 
vity.   The  different  portions  of  the  wing  are  likewise  so  dis- 
posed as  to  be  contracted  and  folded  together  when  the  wing 
is  drawn  up,  but  fully  expanded  when  it  descends  in  order 
to  strike  the  air.     It  is  obvious  that,  without  this  provision, 
a  great  part  of  the  motion  acquired  by  the  resistance  of  the 
air  against  the  wing  in  its  descent  would  have  been  lost  by 
a  counteracting  resistance  during  its  ascent.     The  disposi- 
tion of  the  great  feathers  is  such  that  they  strike  the  air  with 
their  flat  sides,  but  present  only  their  edges  in  rising;  what 
is  c2L\\edi  feathering  the  oar  in  rowing  is  a  similar  operation, 
performed  with  the  same  intention,  and  deriving  its  name 
from  this  resemblance. 

As   the   inclination    of  the    wing  is  chiefly    backw^ards, 
the  greatest  part   of  the  effect  produced  by  its   action  is 
to  move  the  body  forwards.     Birds  of  prey  have  a  great 
obliquity  of  wing,  and  are  consequently  better  formed  for 
horizontal  progressive  motion,  which   is  what  they  chiefly 
practise  in  pursuing  their  prey,  than  for  a  rapid  perpendi- 
cular ascent.     Those  birds,  on  the  contrary,  which  rise  to 
great  heights  in  a  direction  nearly  vertical,  such  as  the  Quail 
and  the  Lark,  have  the  wings  so  disposed  as  to  strike  di- 
rectly downwards,  without  any  obliquity  whatsoever.     For 
the  same  reason,  birds  rise  better  against  the  wind,  which, 
acting  upon    the  oblique  surface  presented  by  the    wings 
during  their  flexion,  contributes  to  the  ascent  of  the  body 
on  the  same  principle  that  a  kite  is   carried  up  into  the  air 
when  retained  in  an  oblique  position.     This  circumstance 
is  particularly  observable  in  the  ascent  of  birds  of  prey, 
whose  wings  have  a  great  obliquity,  and,  when  fully  expand- 
ed, present  a  very  large  extent  of  surface. 

Vol.  I.  51 


402  THE  MECHANICAL  FUNCTIONS. 

The  actions  of  the  tail,  which  operates  as  a  rudder,  are 
useful  chiefly  in  directing  the  flight.  When  the  tail  is  short, 
this  office  is  supplied  by  the  legs,  which  are  in  that  case 
generally  very  long;  and  being  raised  high  and  extended 
backwards  in  a  straight  line,  are  of  considerable  assistance 
in  the  steerage  of  the  animal.  In  many  birds,  as  in  the 
wood-pecker,  the  tail  is  much  employed  as  a  support  to  the 
body  in  climbing  trees.  The  caudal  vertebrae  are  often  nu- 
merous, but  are  short  and  compressed  together;  they  are  re- 
markable for  the  great  development  of  their  transverse  pro- 
cesses, and  for  having  spinous  processes  both  on  their  lower 
and  upper  sides.  The  last  vertebra,  instead  of  being  cylin- 
drical, has  a  broad  carinated  spine  for  the  insertion  of  large 
feathers. 

Birds  could  not,  of  course,  be  always  on  the  wing;  for  a 
great  expenditure  of  muscular  power  is  constantly  going  on 
while  they  support  themselves  in  the  air.  Occasional  rest 
is  necessary  to  them  as  well  as  to  other  animals,  and  means 
are  accordingly  provided  by  nature  for  their  mechanical 
support  and  progressive  motion  while  on  land. 

The  anterior  extremities  having  been  exclusively  appro- 
priated to  flight,  and  constructed  with  reference  to  the  pro- 
perties of  the  atmosphere,  the  offices  of  sustaining  and  of 
moving  the  body  along  the  ground  must  be  intrusted  wholly 
to  the  hind  limbs.     The  centre  of  gravity,  before  sustained 
by  the  wings,  must  now  be  brought  over  the  new  basis  of 
support  formed  by  the  feet;  or  rather,  as  it  is  placed  far  for- 
wards, the  feet  must  be  considerably  advanced  so  as  to  be 
brought  underneath  that  centre.     But  as  the  bones  of  the 
posterior  extremity  have  their  origin  from  the  remote  part 
of  the  pelvis,  which  is  elongated  backwards,  at  a  considera- 
ble distance  from  the  wings,  it  became  necessary  to  lengthen 
some  of  their  parts,  and  to  bend  their  joints  at  very  acute 
angles.     We  accordingly  find  that  while  nature,  in  the  for- 
mation of  the  limb,  has  preserved  an  accordance  with  the 
vertebrated  type,  both  as  to  the  number  of  pieces  which 
compose  it,  and  as  to  their  relative  situations,  she  has  devi- 


FEET  OF  BIRDS.  403 

ated  from  the  model  of  quadrupeds  in  giving  much  greater 
length  to  the  division  corresponding  to  the  foot.  At  the 
same  time  that  the  foot  is  brought  forwards,  the  toes  are 
lengthened,  and  made  to  spread  out  so  as  to  enclose  a  wide 
base,  over  which  the  centre  of  gravity  is  situated.  The  ex- 
tent of  this  base  is  so  considerable  that  a  bird  can,  in  general, 
support  itself  with  ease  upon  a  single  foot,  without  danger  of 
being  overset  by  the  unavoidable  vacillations  of  its  body. 

The  femur  is  short  compared  with  the  tibia,  which  is  ge- 
nerally large,  especially  in  the  order  of  Grallx^  or  wading 
birds:  the  fibula  is  exceedingly  slender,  and  always  united, 
at  its  lower  part,  with  the  tibia;  and  there  is  a  total  deficien- 
cy of  tarsal  bones,  except  in  the  Ostrich,  where  rudiments 
of  them  may  be  traced.  Already  we  have  seen,  in  ruminant 
quadrupeds,  that  these  bones  have  dwindled  to  a  very  small 
size,  but  here  they  have  wholly  disappeared.  The  long 
bone  which  succeeds  to  the  tibia,  though  considered  by  some 
anatomists  as  the  tarsus,  is,  properly,  the  metatarsal  bone,  and 
in  the  Grallcc  is  of  great  length.  At  its  lower  end  it  has 
three  articulations,  shaped  like  pulleys,  for  the  attachment  of 
the  three  toes:  there  is,  besides,  in  almost  all  birds,  a  small 
rudiment  of  another  metatarsal  bone,  on  which  is  situated 
the  fourth  toe.  The  number  of  bones  which  compose  each 
respective  toe  appears  to  be  regulated  by  a  uniforjn  law. 
The  innermost  toe,  which  may  be  compared  to  a  thumb,  con- 
sists invariably  of  two  bones:  that  which  is  next  to  it  in  the 
order  of  sequence  has  always  three;  that  which  follows  has 
four;  and  the  outermost  toe  has  five  bones:  the  claws  in  eve- 
ry case  being  affixed  to  the  last  joints,  which  have,  therefore, 
been  termed  the  ungual  bones.  This  remarkable  numerical 
relation,  among  the  several  bones  of  the  toes,  exists  quite  in- 
dependently of  their  length. 

There  is  one  whole  order  of  ])irds  which  are  particularly 
fitted  for  climbing  and  perching  upon  trees,  having  the  two 
middle  toes  parallel  to  each  other,  and  the  inner  and  outer 
toes  turned  back,  so  as  to  be  opposed  to  them  in  their  action. 
They  are  thus  enabled  to  grasp  objects  with  the  greatest  fa- 


404  THE  MECHANICAL  FUNCTIONS. 

cility;  having,  in  fact,  two  thumbs,  which  are  opposable  to 
the  two  fingers.     They  have  been  termed  Scansores,  or  Zi/-  • 
godactyli.     Almost  all  other  birds  have  three  toes  before, 
and  one  behind. 

From  this  enumeration,  it  would  appear  as  if  Nature,  in 
modifying  the  type  of  vertebrated  animals  to  suit  the  pur- 
poses required  in  the  bird,  had  purposely  omitted  one  of  the 
toes,  which  are  usually  five  in  number.  But  instances  occur 
of  birds,  in  which  we  may  trace  the  rudiment  of  a  fifth  toe 
high  upon  the  metatarsus,  and  upon  its  inner  side.  The 
spur  of  the  cock  may  be  regarded  as  having  this  origin. 
What  confirms  this  view  of  the  subject,  is,  that  in  those 
birds  which  have  only  three  toes,  namely,  in  the  Emit,  the 
Cassowary,  and  the  Rhea,  it  is  again  the  inner  toe  which 
disappears,  leaving  only  the  three  outer  toes,  namely,  those 
which  have,  respectively,  three,  four,  and  five  phalanges. 
The  Ostrich  has  only  two  toes,  one  having  four,  and  the 
other  five  phalanges;  here,  again,  it  is  the  innermost  of  the 
three  former,  that  is,  the  one  having  three  phalanges,  which 
has  been  suppressed.* 

A  bird  is  capable  of  shifting  the  position  of  the  centre  of 
gravity  of  its  body  according  as  circumstances  require  it, 
simply  by  advancing  or  drawing  back  its  head.  While  fly- 
ing, the  neck  is  stretched  forwards  to  the  utmost,  in  order 
to  bring  the  centre  of  gravity  immediately  under  the  origin 
of  the  wings,  by  which  the  body  is  then  suspended.  When 
birds  stand  upon  their  feet,  they  carry  the  head  back  as 
far  as  possible;  so  as  to  balance  the  body  on  the  base  of  sup- 
port. When  preparing  to  sleep,  they  bring  the  centre  of 
gravity  still  lower,  by  turning  the  head  round  and  placing 
it  under  the  w^ing.  These  motions  of  the  head  are  again  re- 
sorted to  when  the  bird  walks;  and  the  centre  of  gravity  is 
thus  transferred  alternately  from  one  foot  to  the  other:  hence, 

*  The  last  bone  of  the  outer  toe  of  the  ostrich  is  very  small,  and  being 
usually  lost  in  preparing  the  skeleton,  has  been  overlooked  by  naturalists; 
but  Dr.  Grant  has  ascertained,  by  the  careful  dissection  of  a  recent  specimen, 
the  existence  of  tiiis  fifth  phalanx. 


FEET  OP  BIRDS. 


405 


in  walking,  the  head  of  a  bird  is  in  constant  motion:  whilst 
the  duck  and  other  birds,  whose  legs  are  very  short,  have  a 
waddling  gait.  It  may  be  observed  that  the  more  perfect- 
ly predaceous  birds  are  not  the  best  formed  for  walking;  be- 
cause where  they  use  their  feet  for  that  purpose,  their  talons, 
which  are  required  to  be  kept  sharp  for  seizing  and  tearing 
their  prey,  would  be  blunted;  and  accordingly  the  eagle, 
when  moving  along  the  ground,  supports  itself  partly  by  the' 
motion  of  its  win^s. 

In  roosting,  birds  support  themselves  upon  their  perch 
by  means  of  one  leg  only,  the  other  being  folded  close  to 
the  body.  They  even  maintain  this  attitude  with  greater 
ease  and  security  than  if  they  rested  upon  both  feet.  '  The 
true  explanation  of  this  curious  fact  was  long  ago  given  by 
Borelli.  On  tracing  the  tendons  (t,  t  Fig.  233)  of  the  mus- 
cles (m,  m)  which  bend  the  claws,  and  enable  them  to  grasp 
an  object,  we  find  them  passing  over  the  outer  angles  of  each 
of  the  intervening  joints,  so  that  whenever  these  joints  are 
bent,  as  shown  in  Fig.  234,  those  tendons  are  put  upon  the 


stretch,  and  mechanically,  or  without  any  action  of  the  inus- 
cles,  tend  to  close  the  foot.  When  the  bird  is  on  its  perch, 
this  effect  is  produced  by  the  mere  weight  of  the  body, 
which  of  course,  tends  to  bend  all  the  joints  of  the  limb  on 
which  it  rests;  so  that  the  greater  that  weight,  the  greater  is 


406  THE  MECHANICAL  FUNCTIONS. 

the  force  with  which  the  toes  grasp  the  perch.  All  this 
takes  place  without  muscular  effort  or  volition  on  the  part 
of  the  bird.  It  remains  in  this  position  with  more  security 
on  one  foot  than  it  would  have  done  by  resting  upon  both; 
because,  in  the  latter  case,  the  weight  of  the  body  being  di- 
vided between  them,  does  not  stretch  the  tendons  sufficient- 
ly. In  this  position,  the  bird  not  only  sleeps  in  perfect  se- 
curity, but  resists  the  impulse  of  the  wind  and  the  shaking 
of  the  bough. 

The  great  length  of  the  toes  of  birds  enables  them  to  stand 
steadily  on  one  leg:  and  in  this  attitude  many  employ  the 
other  foot  as  a  hand;  especially  parrots,  whose  head  is  too 
heavy  to  be  readily  brought  to  the  ground.  Some  birds, 
which  frequent  the  banks  of  rivers,  are  in  the  practice  of 
holding  a  stone  in  one  foot,  while  they  rest  upon  the  other: 
this  contributes  to  increase  their  stability  in  two  ways;  first, 
it  adds  to  the  weight  of  the  body,  which  is  the  force  that 
stretches  the  tendons,  and  causes  them  to  grasp  the  bough; 
and,  secondly,  it  also  lowers  the  centre  of  gravity. 

The  stork,  and  some  other  birds  belonging  to  the  same 
order,  which  sleep  standing  on  one  foot,  have  a  curious  me- 
chanical contrivance  for  locking  the  joint  of  the  tarsus,  and 
preserving  the  leg  in  a  state  of  extension  without  any  mus- 
cular effort.  The  mechanism  is  such  as  to  withstand  the  ef- 
fect of  the  ordinary  oscillations  of  the  body,  when  the  bird 
is  reposing;  but  it  is  easily  unlocked  by  a  voluntary  muscu- 
lar exertion,  when  the  limb  is  to  be  bent  for  progression. 
On  these  occasions  the  ball  of  the  metatarsal  bone  is  driven 
with  some  force  into  the  socket  of  the  tibia.* 

I  must  content  myself  with  this  general  view  of  the  me- 
chanism of  birds;  as  it  would  exceed  the  limits  within  which 
I  must  confine  myself,  to  enter  more  fully  into  the  peculi- 
arities which  distinguish  the  different  orders  and  families. 

*  This  mechanism  is  noticed  by  Dr.  Macartney,  in  the  Transactions  of  the 
Royal  Irish  Academy,  vol.  xiii.  p.  20,  and  is  more  fully  described  in  Rees's 
Cyclopjedia,  Art.  Bird.  He  observes  that  both  Cavier  and  Dumeril  have 
committed  an  en-or  in  referring  this  peculiarity  of  structure  to  the  knee  in- 
stead of  the  tarsal  joint. 


FEET  OF  BIRDS.  407 

Some  of  the  more  remarka!)lc  deviations  from  what  may  be 
considered  as  the  standard  conformation,  may,  however,  for 
a  moment  arrest  our  attention. 

The    Ostrich  is  of  all  birds  the  one  that  presents  the 
greatest  number  of  exceptions  to  the  general  rules  which 
appear  to  regulate  the  conformation  of  birds,  and  in  many 
of  its  peculiarities  of  structure  it  makes  some  approach  to 
that  which  characterizes  the  quadruped.     Though  this  bird 
is  provided  with  wnngs,.  it  was  evidently  never  intended 
that  they  should  be  used  for  the  purposes  of  flight.     Hence 
the  chief  muscular  power  has  been  bestowed  on  the  legs, 
which  are  remarkably  thick  and  strong,  and  well  fitted  for 
rapid  progression.    The  sternum  is  flat,  and  does  not  present 
the  keel-like  projection  which  is  so  remarkable  in  that  of  all 
other  birds.     The  clavicles  do  not  reach  the  sternum,  nor 
even  meet  at  the  anterior  part  of  the  chest  to  form  the  fur- 
cular  bone:  for  as  the  wings  are  not  employed  in  flying,  the 
usual  office  of  that  bone  is  not  wanted.     The  form  of  the 
pelvis  is  diflerent  from  the  ordinary  structure;  for  the  pubic 
bones,  which  in  all  other  birds  are  separated  by  an  interval, 
here  unite  as  they  do  in  quadrupeds. 

The  feathers  are  unprovided  with  that  elaborate  apparatus 
of  crotchets  and  fibres,  which  are  universally  met  with  in 
birds  that  fly.  The  filaments  of  the  ostrich's  feathers,  in 
consequence  of  having  none  of  these  fibrils,  hang  loose  and 
detached  from  one  another,  forming  the  fine  hair  or  down^ 
which,  however  ornamental  as  an  article  of  dress,  must  be 
viewed,  when  considered  physiologically,  as  a  species  of  de- 
generacy in  the  structure  of  feathers. 

The  Penguin,  in  like  manner,  has  a  wing,  which  is,  by  its 
shortness,  totally  unfitted  for  raising  the  body  in  the  air:  it 
has,  indeed,  received  a  very  diflferent  destination,  being 
formed  for  swimming.  In  external  form,  it  resembles  the 
anterior  extremity  of  the  turtle;  but,  still,  we  find  it  con- 
structed on  the  model  of  the  wings  of  birds;  as  if  nature  had 
bound  herself,  by  a  law,  not  to  depart  from  the  standard  of 
organization,  although  the  purpose  of  the  structure  is  alto- 
gether changed.     As  penguins  are  intended  for  a  maritime 


408  THE   MECHANICAL  FUNCTIONS. 

life,  all  their  extremities  are  formed  for  swimming.  Their 
legs  are  exceedingly  short,  and  placed  far  backwards;  so  that 
these  birds  are  compelled,  when  resting  on  their  feet  on  the 
shore,  to  raise  their  bodies  in  a  perpendicular  attitude,  in 
order  to  place  the  centre  of  gravity  immediately  above  the 
base  of  support:  a  posture  which  gives  them  a  strange  and 
grotesque  appearance. 

I  have  already  alluded  to  the  lengthened  legs  and  feet  of 
the  waders,  the  utility  of  which  to  birds  frequenting  marshy 
places,  and  shallow  waters,  is  very  obvious.  Their  legs  are 
not  covered  with  feathers,  which  would  have  been  injured 
by  continual  exposure  to  wet.  But  birds  of  a  truly  aquatic 
nature  have  their  toes  webbed,  that  is,  united  by  a  mem- 
brane, a  mechanism  which  qualifies  them  to  act  as  oars,  and, 
indeed,  gives  them  a  great  advantage  over  all  artificial  oars, 
that  have  been  constructed  by  human  ingenuity;  for,  as  soon 
as  the  expanded  foot  has  impelled  the  water  behind  it,  the 
toes  collapse,  and,  while  it  is  drawn  forward,  it  presents  a 
very  small  surface  to  the  opposing  water.  Their  plumage 
is  so  constructed  as  to  prevent  the  water  from  penetrating 
through  it;  and  for  the  purpose  of  preserving  it  in  this  con- 
dition, these  birds  are  provided  with  an  oily  fluid,  which 
they  carefully  spread  over  the  whole  surface  of  their  bodies. 
The  Swan,  and  many  other  water-fowls,  employ  their  wings 
as  sails,  and  are  carried  forwards  on  the  water  with  consi- 
derable velocity,  by  the  impulse  of  the  wind. 

Birds  excel  all  other  vertebrated  animals  in  the  energy  of 
their  muscular  powers.  The  promptitude,  the  force,  and 
the  activity  they  display  in  all  their  movements,  and  the  un- 
wearied vigour  with  which  they  persevere,  for  hours  and 
days,  in  the  violent  exertions  required  for  flight,  far  exceed 
those  of  any  quadruped,  and  implies  a  higher  degree  of  irri- 
tability, dependent,  probably,  on  the  great  extent  of  their 
respiratory  functions,  than  is  possessed  by  any  other  class  of 
animals. 


ST^^ 


END  OF  VOL.   I. 


.1 

J 

i 


1^ 


) 


^ 


/^.i:^* 


/ 


